urlmatch.c: recompute pointer after append_normalized_escapes

When append_normalized_escapes is called, its internal strbuf_add* calls can
cause the strbuf's buf to be reallocated changing the value of the buf pointer.

Do not use the strbuf buf pointer from before any append_normalized_escapes
calls afterwards.  Instead recompute the needed pointer.

Signed-off-by: Thomas Rast <trast@inf.ethz.ch>
Signed-off-by: Kyle J. McKay <mackyle@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>