Check for NULL in prefix_dir.

This commit fixes trac:1337.

This commit was SVN r18612.

The following Trac tickets were found above:
  Ticket 1337 --> https://svn.open-mpi.org/trac/ompi/ticket/1337

orte/mca/odls/base/odls_base_default_fns.c

@@ -573,7 +573,7 @@ static int odls_base_default_setup_fork(orte_app_context_t *context,
         find PATH or LD_LIBRARY_PATH.  If found, that means the
         prior work was clobbered, and we need to re-prefix those
         variables. */
-    for (i = 0; NULL != context->env && NULL != context->env[i]; ++i) {
+    for (i = 0; NULL != context->prefix_dir && NULL != context->env && NULL != context->env[i]; ++i) {
         char *newenv;
         
         /* Reset PATH */