SDL_RWFromMem, SDL_RWFromConstMem: reject negative size parameters.

(cherry picked from commit 0ad822eb70)

src/file/SDL_rwops.c

@@ -648,7 +648,7 @@ SDL_RWops *SDL_RWFromMem(void *mem, int size)
         SDL_InvalidParamError("mem");
         return rwops;
     }
-    if (!size) {
+    if (size <= 0) {
         SDL_InvalidParamError("size");
         return rwops;
     }
@@ -675,7 +675,7 @@ SDL_RWops *SDL_RWFromConstMem(const void *mem, int size)
         SDL_InvalidParamError("mem");
         return rwops;
     }
-    if (!size) {
+    if (size <= 0) {
         SDL_InvalidParamError("size");
         return rwops;
     }