Fixed exec failure regression

The process_t pointer sent to setup_child_process can actually be 0
without it being failure, as that is what fish sends when `exec` is run
(in the case of INTERNAL_EXEC).

This was causing exec to fail.

src/postfork.cpp

@@ -250,8 +250,9 @@ static int handle_child_io(const io_chain_t &io_chain) {
 }
 
 int setup_child_process(job_t *j, process_t *p, const io_chain_t &io_chain) {
-    bool ok = false;
+    bool ok = true;
 
+    //p is zero when EXEC_INTERNAL
     if (p) {
         ok = child_set_group(j, p);
     }