Fix segfault in call to ngx_read_file of partial key purge

Because file.log is not initialized and the file variable is not zeroed
out after initialization, the call to ngx_log_error_core in
ngx_read_file results in a segmentation fault.

Author: fdintino

ngx_cache_purge_module.c

@@ -1249,9 +1249,11 @@ ngx_http_purge_file_cache_delete_partial_file(ngx_tree_ctx_t *ctx, ngx_str_t *pa
     } else {
         ngx_file_t file;
 
+        ngx_memzero(&file, sizeof(ngx_file_t));
         file.offset = file.sys_offset = 0;
         file.fd = ngx_open_file(path->data, NGX_FILE_RDONLY, NGX_FILE_OPEN,
                                 NGX_FILE_DEFAULT_ACCESS);
+        file.log = ctx->log;
 
         /* I don't know if it's a good idea to use the ngx_cycle pool for this,
            but the request is not available here */