Fix invalid read in quote_c_style_counted

Fix invalid read in quote_c_style_counted

This function did not work on strings that were not NUL-terminated. It
reads through a length-bounded string, searching for characters in need of
quoting. After we find one, we output the quoted character, then advance
our pointer to find the next one. However, we never decremented the
length, meaning we ended up looking at whatever random junk was stored
after the string.

This bug was not found by the existing tests because most code paths feed
a NUL-terminated string. The notable exception is a directory name being
fed by ls-tree.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>