color: protect against out-of-bounds reads and writes

want_color_fd() is designed to work only with standard output and
error file descriptors and stores information about each descriptor in
an array. However, it doesn't verify that the passed-in descriptor
lives within that set, which, with a buggy caller, could lead to
access or assignment outside the array bounds.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Acked-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

color.c

@@ -343,6 +343,9 @@ int want_color_fd(int fd, int var)
 
 	static int want_auto[3] = { -1, -1, -1 };
 
+	if (fd < 1 || fd >= ARRAY_SIZE(want_auto))
+		BUG("file descriptor out of range: %d", fd);
+
 	if (var < 0)
 		var = git_use_color_default;