Core: fixed memory leak on configuration reload with PCRE2.

In ngx_regex_cleanup() allocator wasn't configured when calling
pcre2_compile_context_free() and pcre2_match_data_free(), resulting
in no ngx_free() call and leaked memory.  Fix is ensure that allocator
is configured for global allocations, so that ngx_free() is actually
called to free memory.

Additionally, ngx_regex_compile_context was cleared in
ngx_regex_module_init().  It should be either not cleared, so it will
be freed by ngx_regex_cleanup(), or properly freed.  Fix is to
not clear it, so ngx_regex_cleanup() will be able to free it.

Reported by ZhenZhong Wu,
https://mailman.nginx.org/pipermail/nginx-devel/2023-September/3Z5FIKUDRN2WBSL3JWTZJ7SXDA6YIWPB.html

src/core/ngx_regex.c

@@ -600,6 +600,8 @@ ngx_regex_cleanup(void *data)
      * the new cycle, these will be re-allocated.
      */
 
+    ngx_regex_malloc_init(NULL);
+
     if (ngx_regex_compile_context) {
         pcre2_compile_context_free(ngx_regex_compile_context);
         ngx_regex_compile_context = NULL;
@@ -611,6 +613,8 @@ ngx_regex_cleanup(void *data)
         ngx_regex_match_data_size = 0;
     }
 
+    ngx_regex_malloc_done();
+
 #endif
 }
 
@@ -706,9 +710,6 @@ ngx_regex_module_init(ngx_cycle_t *cycle)
     ngx_regex_malloc_done();
 
     ngx_regex_studies = NULL;
-#if (NGX_PCRE2)
-    ngx_regex_compile_context = NULL;
-#endif
 
     return NGX_OK;
 }