rerere.c: diagnose a corrupt MERGE_RR when hitting EOF between TAB and '\0'

If we reach EOF after the SHA1-then-TAB, yet before the NUL that
terminates each file name, we would fill the file name buffer with \255
bytes resulting from the repeatedly-failing fgetc (returns EOF/-1) and
ultimately complain about "filename too long", because no NUL was
encountered.

Signed-off-by: Jim Meyering <jim@meyering.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

rerere.c

@@ -42,8 +42,14 @@ static void read_rr(struct string_list *rr)
 		name = xstrdup(buf);
 		if (fgetc(in) != '\t')
 			die("corrupt MERGE_RR");
-		for (i = 0; i < sizeof(buf) && (buf[i] = fgetc(in)); i++)
-			; /* do nothing */
+		for (i = 0; i < sizeof(buf); i++) {
+			int c = fgetc(in);
+			if (c < 0)
+				die("corrupt MERGE_RR");
+			buf[i] = c;
+			if (c == 0)
+				 break;
+		}
 		if (i == sizeof(buf))
 			die("filename too long");
 		string_list_insert(rr, buf)->util = name;