AP_HAL_SITL: Fix bad check order on heap_realloc

libraries/AP_HAL_SITL/Util.cpp

@@ -95,14 +95,14 @@ void *HALSITL::Util::heap_realloc(void *heap_ptr, void *ptr, size_t new_size)
         old_size = old_header->allocation_size;
     }
 
-    heapp->current_heap_usage -= old_size;
-    if (new_size == 0) {
-       free(old_header);
+    if ((heapp->current_heap_usage + new_size - old_size) > heapp->scripting_max_heap_size) {
+        // fail the allocation as we don't have the memory. Note that we don't simulate fragmentation
         return nullptr;
     }
 
-    if ((heapp->current_heap_usage + new_size - old_size) > heapp->scripting_max_heap_size) {
-        // fail the allocation as we don't have the memory. Note that we don't simulate fragmentation
+    heapp->current_heap_usage -= old_size;
+    if (new_size == 0) {
+       free(old_header);
        return nullptr;
     }