ess/singleton: only realloc() when required in fork_hnp()

orte/mca/ess/singleton/ess_singleton_module.c

@@ -580,10 +580,14 @@ static int fork_hnp(void)
             }
             /* we read something - better get more */
             num_chars_read += rc;
+            chunk -= rc;
+            if (0 == chunk) {
+                chunk = ORTE_URI_MSG_LGTH;
                 orted_uri = realloc((void*)orted_uri, buffer_length+chunk);
                 memset(&orted_uri[buffer_length], 0, chunk);
                 buffer_length += chunk;
             }
+        }
         close(p[0]);
 
         if (num_chars_read <= 0) {