dl dlopen: ensure dirs is not NULL

opal_argv_split() may have returned NULL.

This was Coverity CID 1288088

opal/mca/dl/dlopen/dl_dlopen_module.c

@@ -162,7 +162,7 @@ static int dlopen_foreachfile(const char *search_path,
     char **good_files = NULL;
 
     dirs = opal_argv_split(search_path, OPAL_ENV_SEP);
-    for (int i = 0; NULL != dirs[i]; ++i) {
+    for (int i = 0; NULL != dirs && NULL != dirs[i]; ++i) {
 
         dp = opendir(dirs[i]);
         if (NULL == dp) {