Fix memcpy misuse in TTF_DeleteTextString

AddressSanitizer aborts the program when calling TTF_DeleteTextString,
which in turn calls SDL_memcpy instead of the correct SDL_memmove.

This commit fixes the abort under ASan.

(cherry picked from commit 8af772a)

Author: mcendu

src/SDL_ttf.c

@@ -5073,7 +5073,7 @@ bool TTF_DeleteTextString(TTF_Text *text, int offset, int length)
         text->text[offset] = '\0';
     } else {
         int shift = (old_length - length - offset);
-        SDL_memcpy(&text->text[offset], &text->text[offset + length], shift);
+        SDL_memmove(&text->text[offset], &text->text[offset + length], shift);
         text->text[offset + shift] = '\0';
     }