Always create a full 256-entry map in case color values are out of range

Fixes https://github.com/libsdl-org/SDL/issues/5042

src/video/SDL_pixels.c

@@ -947,7 +947,7 @@ Map1to1(SDL_Palette * src, SDL_Palette * dst, int *identical)
         }
         *identical = 0;
     }
-    map = (Uint8 *) SDL_malloc(src->ncolors);
+    map = (Uint8 *) SDL_calloc(256, sizeof(Uint8));
     if (map == NULL) {
         SDL_OutOfMemory();
         return (NULL);
@@ -971,7 +971,7 @@ Map1toN(SDL_PixelFormat * src, Uint8 Rmod, Uint8 Gmod, Uint8 Bmod, Uint8 Amod,
     SDL_Palette *pal = src->palette;
 
     bpp = ((dst->BytesPerPixel == 3) ? 4 : dst->BytesPerPixel);
-    map = (Uint8 *) SDL_malloc(pal->ncolors * bpp);
+    map = (Uint8 *) SDL_calloc(256, bpp);
     if (map == NULL) {
         SDL_OutOfMemory();
         return (NULL);