Core: fixed ngx_pcre_studies cleanup.

Core: fixed ngx_pcre_studies cleanup.

If a configuration parsing fails for some reason, ngx_regex_module_init()
is not called, and ngx_pcre_studies remained set despite the fact that
the pool it was allocated from is already freed.  This might result in
a segmentation fault during runtime regular expression compilation, such
as in SSI, for example, in the single process mode, or if a worker process
died and was respawned from a master process in such an inconsistent state.

Fix is to clear ngx_pcre_studies from the pool cleanup handler (which is
anyway used to free JIT-compiled patterns).