Prevent div by zero crash in mana widget

Prevents division by zero crash when displaying widgets with defined
break levels.

Example of where previous crash from div-by-zero happened:
* Provided save used the `zenfs_thick_sidebar`, which includes widget
  `zenfs_mana_bar_14_no_label` that defines break levels.
* When running the provided save, mana value is usually at `v=201` with
  `min=0` `max=201`.
* Those values are computed by `known_magic::max_mana` where internal
  values are `bionic_penalty=999` `mana_base=1000` `int_bonus=200`.
* But when int falls (because of tiredness), then
  `known_magic::max_mana` will return `0` because `int_bonus=-100`.
* This leads to `_var_max=0` in `widget::value_color` and thus
  `var_range=0` which causes the div by zero.

Backtrace of previous crash beng fixed:
```
Thread 1 "cataclysm-tiles" received signal SIGFPE, Arithmetic exception.
0x0000555556a9c54e in widget::value_color (this=this@entry=0x7fffffffb8d8, value=value@entry=201) at src/widget.cpp:1310
1310            const int value_offset = ( 100 * ( value - _var_min ) ) / var_range;

(gdb) bt
 #0  0x0000555556a9c54e in widget::value_color (this=this@entry=0x7fffffffb8d8, value=value@entry=201) at src/widget.cpp:1310
 CleverRaven#1  0x0000555556aa37dc in widget::color_value_string[abi:cxx11](int, int) (this=this@entry=0x7fffffffb8d8, value=201, width_max=width_max@entry=14) at src/widget.cpp:1262
 CleverRaven#2  0x0000555556aa38ca in widget::show[abi:cxx11](avatar const&, unsigned int) (this=this@entry=0x7fffffffb8d8, ava=..., max_width=max_width@entry=14) at src/widget.cpp:890
 CleverRaven#3  0x0000555556aa497f in widget::layout[abi:cxx11](avatar const&, unsigned int, int, bool) (this=this@entry=0x7fffffffb8d8, ava=..., max_width=<optimized out>, label_width=label_width@entry=0, skip_pad=false) at src/widget.cpp:1834
 CleverRaven#4  0x0000555556aa42c2 in widget::layout[abi:cxx11](avatar const&, unsigned int, int, bool) (this=this@entry=0x7fffffffbe88, ava=..., max_width=<optimized out>, label_width=label_width@entry=9, skip_pad=false) at src/widget.cpp:1794
 CleverRaven#5  0x0000555556aa3cfa in widget::layout[abi:cxx11](avatar const&, unsigned int, int, bool) (this=this@entry=0x7fffffffc438, ava=..., max_width=<optimized out>, label_width=label_width@entry=0, skip_pad=false) at src/widget.cpp:1724
 CleverRaven#6  0x0000555556aa42c2 in widget::layout[abi:cxx11](avatar const&, unsigned int, int, bool) (this=this@entry=0x7fffffffc9e8, ava=..., max_width=<optimized out>, label_width=label_width@entry=0, skip_pad=false) at src/widget.cpp:1794
 CleverRaven#7  0x0000555556aa42c2 in widget::layout[abi:cxx11](avatar const&, unsigned int, int, bool) (this=this@entry=0x555562c47160, ava=..., max_width=<optimized out>, max_width@entry=64, label_width=<optimized out>, skip_pad=skip_pad@entry=false) at src/widget.cpp:1794
 CleverRaven#8  0x0000555556aa5222 in custom_draw_func (args=...) at src/widget.cpp:970
 CleverRaven#9  0x0000555555ec4360 in std::function<int(draw_args const&)>::operator() (this=0x555597eb1780, __args#0=...) at /usr/include/c++/14/bits/std_function.h:591
 CleverRaven#10 game::draw_panels (this=this@entry=0x55555827ef50, force_draw=force_draw@entry=true) at src/game.cpp:4007
 CleverRaven#11 0x0000555555eed1ac in game::draw (this=0x55555827ef50, ui=...) at src/game.cpp:3961
 CleverRaven#12 0x00005555569a962c in ui_adaptor::redraw_invalidated () at src/ui_manager.cpp:440
 CleverRaven#13 0x00005555569a96e9 in ui_adaptor::redraw () at src/ui_manager.cpp:345
 CleverRaven#14 0x00005555569a9710 in ui_manager::redraw () at src/ui_manager.cpp:506
 CleverRaven#15 0x0000555555dd0850 in do_turn () at src/do_turn.cpp:712
 CleverRaven#16 0x00005555557a138d in main (argc=<optimized out>, argv=<optimized out>) at src/main.cpp:873

(gdb) print var_range
$1 = 0
(gdb) print value
$2 = 201
(gdb) print _var_min
$3 = 0
(gdb) print _var_max
$4 = 0
```

src/widget.cpp

@@ -1306,7 +1306,7 @@ nc_color widget::value_color( int value )
     // Get range of values from min to max
     const int var_range = _var_max - _var_min;
 
-    if( ! _breaks.empty() ) {
+    if( var_range > 0 && ! _breaks.empty() ) {
         const int value_offset = ( 100 * ( value - _var_min ) ) / var_range;
         for( int i = 0; i < color_max; i++ ) {
             if( value_offset < _breaks[i] ) {