Fixed heap size not being incremented

photon/libc/sys/magenta/heap.c

@@ -36,6 +36,7 @@ void *__extend_heap(size_t sz)
     uintptr_t ret = heap_end;
     mx_segment_set_size(heap_segment, heap_sz + sz);
     mx_segment_map(heap_segment, 0, heap_sz + sz, &start);
+    heap_sz += sz;
     heap_end += sz;
     return (void *)ret;
 }