Replace realloc with g_realloc in icon-lookup.c

[Costinteo]

The purpose is to avoid any failure in realloc, leading to null pointer dereferences/memory leaks. Failure in g_realloc will also terminate Dunst.
@bebehei, waiting for any further suggestions/reviews. :)

[bebehei]

Hey @Costinteo, I've checked the code. The problems you found are fixed, but this PR will solve a fraction of the memory problems.

Why is the calloc in the same file not problematic? See here:

dunst/src/icon-lookup.c

Line 67 in dfab9f0

icon_themes[index].dirs = calloc(icon_themes[index].dirs_count, sizeof(struct icon_theme_dir));

[Costinteo]

@bebehei, following your comment, I've analysed the rest of the code base and replaced all alloc functions with their GLib counterparts.
calloc calls have been specifically replaced with g_malloc followed by a memset to 0. Except for this case, where the memory was initialised anyway, in the next few lines:
https://github.com/Costinteo/dunst/blob/master/src/icon-lookup.c#L130

EDIT: Just noticed there's a g_malloc0 in GLib. I'll be replacing the g_malloc + memset pairs with that, instead.
EDIT2: Done. You can look through it.

[fwsmit]

calloc should be replaced with gmalloc0_n, not gmalloc0, so I fixed that. Thanks for the PR!