Add a paranoia check in string_is_valid_nonrfc_hostname()

The earlier checks in this function should ensure that components is
always nonempty.  But in case somebody messes with them in the
future, let's add an extra check to make sure we aren't crashing.

src/common/util.c

@@ -1125,6 +1125,9 @@ string_is_valid_nonrfc_hostname(const char *string)
 
   smartlist_split_string(components,string,".",0,0);
 
+  if (BUG(smartlist_len(components) == 0))
+    return 0; // LCOV_EXCL_LINE should be impossible given the earlier checks.
+
   /* Allow a single terminating '.' used rarely to indicate domains
    * are FQDNs rather than relative. */
   last_label = (char *)smartlist_get(components,