add -p: avoid use of undefined $key when ReadKey -> EOF

b5cc003253 (add -i: ignore terminal escape sequences, 2011-05-17)
add an additional check to the original code to better handle keys
for escape sequences, but failed to account for the possibility
the first ReadKey call returned undef (ex: stdin closes) and that
was being handled fine by the original code in ca6ac7f135 (add -p:
prompt for single characters, 2009-02-05)

Add a test for undefined and encapsulate the loop and the original
print that relied on it within it.

After this, the following command (in a suitable repository state)
wouldn't print any error:

  $ git -c interactive.singleKey add -p </dev/null

Signed-off-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-add--interactive.perl

@@ -1175,15 +1175,17 @@ sub prompt_single_character {
 		ReadMode 'cbreak';
 		my $key = ReadKey 0;
 		ReadMode 'restore';
-		if ($use_termcap and $key eq "\e") {
-			while (!defined $term_escapes{$key}) {
-				my $next = ReadKey 0.5;
-				last if (!defined $next);
-				$key .= $next;
+		if (defined $key) {
+			if ($use_termcap and $key eq "\e") {
+				while (!defined $term_escapes{$key}) {
+					my $next = ReadKey 0.5;
+					last if (!defined $next);
+					$key .= $next;
+				}
+				$key =~ s/\e/^[/;
 			}
-			$key =~ s/\e/^[/;
+			print "$key";
 		}
-		print "$key" if defined $key;
 		print "\n";
 		return $key;
 	} else {