Prevent asan warning on qsort():

'src/stdlib/SDL_qsort.c:27:5: runtime error: null pointer passed as argument 1, which is declared to never be null`

(cherry picked from commit 2cd583ee13)

src/stdlib/SDL_qsort.c

@@ -30,6 +30,9 @@
 #if defined(HAVE_QSORT)
 void SDL_qsort(void *base, size_t nmemb, size_t size, int (*compare) (const void *, const void *))
 {
+    if (!base) {
+        return;
+    }
     qsort(base, nmemb, size, compare);
 }