res_musiconhold: Don't crash when real-time doesn't return any entries

ASTERISK-29211 #close

Change-Id: Ifbf0a4f786ab2a52342f9d1a1db4c9907f069877

res/res_musiconhold.c

@@ -1613,6 +1613,8 @@ static struct ast_variable *load_realtime_musiconhold(const char *name)
 			char *category = NULL;
 			size_t entry_count = 0;
 
+			/* entries is NULL if there are no results */
+			if (entries) {
 				while ((category = ast_category_browse(entries, category))) {
 					const char *entry = ast_variable_retrieve(entries, category, "entry");
 
@@ -1625,6 +1627,7 @@ static struct ast_variable *load_realtime_musiconhold(const char *name)
 					}
 				}
 				ast_config_destroy(entries);
+			}
 
 			if (entry_count == 0) {
 				/* Behave as though this class doesn't exist */