Fixed the -p parameter handling.

Ensure that the path supplied always ends with a `/' except when empty.
An empty value now corresponds to the current directory instead of `/'.

src/core/nginx.c

@@ -836,7 +836,7 @@ ngx_process_options(ngx_cycle_t *cycle)
         len = ngx_strlen(ngx_prefix);
         p = ngx_prefix;
 
-        if (!ngx_path_separator(*p)) {
+        if (len && !ngx_path_separator(p[len - 1])) {
             p = ngx_pnalloc(cycle->pool, len + 1);
             if (p == NULL) {
                 return NGX_ERROR;