flambda-backend: Fixed ISO C99 warning introduced in #1705 (#1787)

Author: apilatjs

runtime/extern.c

@@ -1199,7 +1199,6 @@ static void add_to_long_value(value *v, intnat x) {
    undo this addition. */
 intnat reachable_words_once(value root, intnat identifier, value sizes_by_root_id,
     intnat *shared_size) {
-  CAMLassert(identifier >= 0);
   struct extern_item * sp;
   intnat size;
   uintnat mark = Invalid, new_mark;
@@ -1209,6 +1208,8 @@ intnat reachable_words_once(value root, intnat identifier, value sizes_by_root_i
   sp = extern_stack;
   size = 0;
 
+  CAMLassert(identifier >= 0);
+
   while (1) {
     if (Is_long(v)) {
       /* Tagged integers contribute 0 to the size, nothing to do */