BUG/MEDIUM: tcpcheck: do not run a regex on an unallocated buffer

BUG/MEDIUM: tcpcheck: do not run a regex on an unallocated buffer

A "tcp-check expect rstring" rule can be evaluated while the input
buffer of the check is not allocated, in which case b_head() returns
NULL, which is not compatible with regex_exec2() that wants a writable
buffer to write a trailing zero, and dies.

By luck, the surrounding cases work by chance because my_memmem()
returns NULL on NULL, and the binary regex works on a trash buffer
(hence always allocated). When using PCRE/PCRE2 it doesn't crash
either because both return an error for a NULL subject, so only a
pure build with libc regex fails.

It's not easy to reproduce, it requires to run many (~2*nproc) parallel
instances of reg-tests/checks/tcp-check_min-recv.vtc several times
(~200) to see it, but it does happend.

The rule now reports no match when no buffer is allocated, which is
what the string and binary cases already do in the same situation

This can be backported to stable versions.