completion: correctly do a signed comparison, to avoid a segfault

Bug existed since commit 30fc197b (a month ago) which changed the
type of 'i' from int to size_t, causing the comparison to do the
wrong thing when 'threshold' is negative.

This fixes https://savannah.gnu.org/bugs/?53722.
Reported-by: Devin Hussey <husseydevin@gmail.com>

src/text.c

@@ -3614,7 +3614,7 @@ void complete_a_word(void)
 				/* The point where we can stop searching for shard. */
 
 		/* Traverse the whole line, looking for shard. */
-		for (i = pletion_x; i < threshold; i++) {
+		for (i = pletion_x; (int)i < threshold; i++) {
 			/* If the first byte doesn't match, run on. */
 			if (pletion_line->data[i] != shard[0])
 				continue;