msvcrt/tests: Remove redundant NULL check before free(). Found by Smatch.

dlls/msvcrt/tests/heap.c

@@ -355,8 +355,7 @@ START_TEST(heap)
     mem = realloc(NULL, 0);
     ok(mem != NULL, "memory not (re)allocated for size 0\n");
 
-    if (mem)
-        free(mem);
+    free(mem);
 
     test_aligned();
 }