add -p: trap Ctrl-D in 'goto' mode

If the user hit Ctrl-D (EOF) while the script was in 'go to hunk?'
mode, it threw an undefined variable error.  Explicitly test for EOF
and have it re-enter the goto prompt loop.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-add--interactive.perl

@@ -994,6 +994,9 @@ sub patch_update_file {
 					}
 					print "go to which hunk$extra? ";
 					$response = <STDIN>;
+					if (!defined $response) {
+						$response = '';
+					}
 					chomp $response;
 				}
 				if ($response !~ /^\s*\d+\s*$/) {