libjob: return on error in unwrap_string()

Problem: if the Flux security context cannot be allocated,
unwrap_string() falls through to flux_security_configure()
instead of returning.

Add missing return.

Author: garlick

src/common/libjob/unwrap.c

@@ -80,6 +80,7 @@ char *unwrap_string (const char *s,
         errprintf (errp,
                    "failed to create security context: %s",
                    strerror (errno));
+        return NULL;
     }
     if (flux_security_configure (sec, NULL) < 0) {
         errprintf (errp,