bson_malloc() aborts on a zero length malloc.

Compilers aren't required to return a ptr for 0 length, they may return NULL instead. Solution is to use same test as used in bson_malloc0 (somebody appears to have looked into this previously but it didn't get applied to bson_malloc).

This is relevant because there is code in the mongodb c driver that expects to get a valid pointer on a 0 length call to malloc (mongoc_set_for_each() is one example). On a susceptible compiler you can't even get past mongoc_client_new() without the patch applied.

Corrected version:

void *
bson_malloc (size_t num_bytes) /* IN */
{
   void *mem = NULL;

   if (BSON_LIKELY (num_bytes)) {
      if (BSON_UNLIKELY (!(mem = gMemVtable.malloc (num_bytes)))) {
         abort ();
      }
   }

   return mem;
}