Handle tags without values

Otherwise a tag with no value would cause a segfault trying to
unescape the NULL tag pointer. This shouldn't happen for the server
tags we parse, but clients could send @+draft/reply with no value.

irc.c

@@ -250,8 +250,12 @@ static struct Message parse(char *line) {
 			char *key = strsep(&tag, "=");
 			for (uint i = 0; i < TagCap; ++i) {
 				if (strcmp(key, TagNames[i])) continue;
+				if (tag) {
 					unescape(tag);
 					msg.tags[i] = tag;
+				} else {
+					msg.tags[i] = "";
+				}
 				break;
 			}
 		}