uniq: always store previous line length

A bug was introduced in the NUL support refactor leading to
the length of the previous line only being saved if the
previous line was shorter than the current line. If triggered
this lead to copying too much data into the previous line buffer.

Behavior before:

	printf '1234\na\n' | ./uniq
	1234
	a
	4

Behavior after:

	printf '1234\na\n' | ./uniq
	1234
	a

uniq.c

@@ -62,9 +62,9 @@ uniqline(FILE *ofp, struct line *l)
 
 	if (l) {
 		if (!prevl.data || l->len >= prevl.len) {
-			prevl.len = l->len;
-			prevl.data = erealloc(prevl.data, prevl.len);
+			prevl.data = erealloc(prevl.data, l->len);
 		}
+		prevl.len = l->len;
 		memcpy(prevl.data, l->data, prevl.len);
 		prevoff = loff;
 	}