Performance optimization: use a temp buffer on the stack instead

Performance optimization: use a temp buffer on the stack instead
of a malloc'ed buffer in regexec() in cases where the required
buffer size is small.

This will help us avoid a malloc/free pair when executing mod_rewrite
rules that use $1/$2/etc to reference sequences in the matched pattern.

Note: I've also submitted this change as a patch for PCRE, but because
the next PCRE release isn't planned until later this year I'm
committing it to the httpd-2.0 copy of PCRE in the meantime.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@94069 13f79535-47bb-0310-9956-ffa450edef68