detect: fixes use-after-free with http.request_header

detect: fixes use-after-free with http.request_header

Ticket: #6441

This keyword and the response one use a multiple inspection buffer.
But the different instances point to the same memory address
that comes from HttpHeaderGetBufferSpace and is not owned
by the transaction, and is rebuilt, which is a functional
bug in itself.

As it gets crafted, it can get reallocated if one header
is over 1024 bytes, while the previous freed pointer will still get
used for the previous headers.