Don't compare pointer against '0', but NULL

src/stdlib/SDL_qsort.c

@@ -414,7 +414,7 @@ static void qsort_nonaligned(void *base, size_t nmemb, size_t size,
   char *first,*last;
   char *pivot=malloc(size);
   size_t trunc=TRUNC_nonaligned*size;
-  assert(pivot);
+  assert(pivot != NULL);
 
   first=(char*)base; last=first+(nmemb-1)*size;
 
@@ -445,7 +445,7 @@ static void qsort_aligned(void *base, size_t nmemb, size_t size,
   char *first,*last;
   char *pivot=malloc(size);
   size_t trunc=TRUNC_aligned*size;
-  assert(pivot);
+  assert(pivot != NULL);
 
   first=(char*)base; last=first+(nmemb-1)*size;
 
@@ -475,7 +475,7 @@ static void qsort_words(void *base, size_t nmemb,
   int stacktop=0;
   char *first,*last;
   char *pivot=malloc(WORD_BYTES);
-  assert(pivot);
+  assert(pivot != NULL);
 
   first=(char*)base; last=first+(nmemb-1)*WORD_BYTES;