parse_text: return if next is UTF8_INVALID

In the underscore case, the next character is retrieved to check
whether the underscore is at a word break. However, if this character
is UTF8_INVALID, the call to parser_pushch will be a noop. This
results in the loop continuing on further than it should. This just
adds a check to see if next is UTF8_INVALID and returns if it is.

Signed-off-by: Brian Ashworth <bosrsf04@gmail.com>

src/main.c

@@ -219,6 +219,9 @@ static void parse_text(struct parser *p) {
 			} else {
 				utf8_fputch(p->output, ch);
 			}
+			if (next == UTF8_INVALID) {
+				return;
+			}
 			parser_pushch(p, next);
 			break;
 		case '+':