fix(memory): realloc must behave like malloc when ptr is nullptr

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>

src/util/memory.cpp

@@ -137,6 +137,8 @@ void * malloc(size_t sz)  {
 }
 
 void * realloc(void * ptr, size_t sz) {
+    if (ptr == nullptr)
+        return malloc(sz);
     size_t old_sz = malloc_size(ptr);
     g_global_memory.dec(old_sz);
     g_thread_memory.dec(old_sz);