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).