irc: return 0 in function irc_protocol_is_numeric_command if the command is NULL or empty

src/plugins/irc/irc-protocol.c

@@ -69,12 +69,16 @@
 int
 irc_protocol_is_numeric_command (const char *str)
 {
+    if (!str || !str[0])
+        return 0;
+
     while (str && str[0])
     {
         if (!isdigit ((unsigned char)str[0]))
             return 0;
         str++;
     }
+
     return 1;
 }