BUG/MAJOR: frontend: initialize capture pointers earlier

Denys Fedoryshchenko reported and diagnosed a nasty bug caused by TCP
captures, introduced in late 1.5-dev by commit 18bf01e ("MEDIUM: tcp:
add a new tcp-request capture directive"). The problem is that we're
using the array of capture pointers initially designed for HTTP usage
only, and that this array was only reset when starting to process an
HTTP request. In a tcp-only frontend, the pointers are not reset, and
if the capture pool is shared, we can very well point to whatever other
memory location, resulting in random crashes when tcp-request content
captures are processed.

The fix simply consists in initializing these pointers when the pools
are prepared.

A workaround for existing versions consists in either disabling TCP
captures in tcp-only frontends, or in forcing the frontends to work in
HTTP mode.

Thanks to Denys for the amount of testing and detailed reports.

This fix must be backported to 1.5.