diff: avoid stack-buffer-read-overrun for very long name

Due to the use of strncpy without explicit NUL termination,
we could end up passing names n1 or n2 that are not NUL-terminated
to queue_diff, which requires NUL-terminated strings.
Ensure that each is NUL terminated.

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

diff-no-index.c

@@ -109,6 +109,7 @@ static int queue_diff(struct diff_options *o,
 				n1 = buffer1;
 				strncpy(buffer1 + len1, p1.items[i1++].string,
 						PATH_MAX - len1);
+				buffer1[PATH_MAX-1] = 0;
 			}
 
 			if (comp < 0)
@@ -117,6 +118,7 @@ static int queue_diff(struct diff_options *o,
 				n2 = buffer2;
 				strncpy(buffer2 + len2, p2.items[i2++].string,
 						PATH_MAX - len2);
+				buffer2[PATH_MAX-1] = 0;
 			}
 
 			ret = queue_diff(o, n1, n2);