Fix MOTD/Credits scrolling bug

[Ezrashaw]

Summary

Bugfixes "Fix MOTD/Credits scrolling bug"

Purpose of change

Fixes bug no. 1 in #58086, scrolling past the very bottom in either the credits or MOTD submenu would continue to scroll internally (but not visually) requiring extra key pushes to begin to scroll up.

Describe the solution

First PR with cpp code so might not be 'correct', need feedback.
We calculate the end of both the MOTD text and Credits text and simply cap the 'sel_line' variable at that position.

Describe alternatives you've considered

None.

Testing

Builds and runs, scrolling now works as intended.

Additional context

1. We might be able to to this better by making the 'mmenu_motd' and 'mmenu_credits' be vectors of strings instead of just a single newline separated string, as far as I can figure this could require changes to 'display_text' (or maybe even the internal API functions) to accept vectors of strings.
2. Pretty sure that that this check is redundant now and a situation where this is actually invoked cannot exist; this check is basically now just done elsewhere.

diff --git a/src/main_menu.cpp b/src/main_menu.cpp
index d9535633df..8f2847a858 100644
--- a/src/main_menu.cpp
+++ b/src/main_menu.cpp
@@ -515,12 +515,6 @@ void main_menu::display_text( const std::string &text, const std::string &title,
     const auto vFolded = foldstring( text, width );
     int iLines = vFolded.size();
 
-    if( selected < 0 || iLines < height ) {
-        selected = 0;
-    } else if( selected >= iLines - height ) {
-        selected = iLines - height;
-    }
-
     fold_and_print_from( w_text, point_zero, width, selected, c_light_gray, text );
 
     draw_scrollbar( w_border, selected, height, iLines, point_south, BORDER_COLOR, true );

[dseguin]

Just a minor correction to appease the clang-tidy test. Looks fine to me otherwise 👍