Fixed return value of SDL_SaveBMP_RW() depending on set errors after NULL input.

If SDL_SaveBMP_RW() was called with NULL passed as SDL_RWops argument, different
values were returned depending on SDL_GetError(). If no error was set before the
call (or SDL_ClearError() was called) then 0 was returned. This is wrong because
nothing was saved. If an error was set before the call then -1 was returned.

This was fixed by directly returning -1 for NULL input instead of deciding based
on SDL_GetError(). No new error is set because this would otherwise override a
maybe more useful error set in SDL_RWFromFile() which is used by SDL_SaveBMP().

src/video/SDL_bmp.c

@@ -531,6 +531,10 @@ SDL_SaveBMP_RW(SDL_Surface * saveme, SDL_RWops * dst, int freedst)
                              format.BitsPerPixel);
             }
         }
+    } else {
+        /* Set no error here because it may overwrite a more useful message from
+           SDL_RWFromFile() if SDL_SaveBMP_RW() is called from SDL_SaveBMP(). */
+        return -1;
     }
 
     if (surface && (SDL_LockSurface(surface) == 0)) {