wpa_supplicant: Add option -I for additional config file

This option can be used only for global parameters that are not going
to be changed from settings.

Signed-off-by: Dmitry Shmidt <dimitrysh@google.com>
Signed-off-by: Iliyan Malchev <malchev@google.com>

src/utils/os_unix.c

@@ -213,6 +213,9 @@ char * os_rel2abs_path(const char *rel_path)
 	size_t len = 128, cwd_len, rel_len, ret_len;
 	int last_errno;
 
+	if (!rel_path)
+		return NULL;
+
 	if (rel_path[0] == '/')
 		return os_strdup(rel_path);