QuickJS: fixed fetch error-handling typos

QuickJS: fixed fetch error-handling typos

Returned the JS_ThrowOutOfMemory() result from
ngx_qjs_fetch_response_ctor() on ngx_list_init() failure: previously
the call result was discarded and execution fell through.

Replaced a stale "ret" check with "rc" after ngx_qjs_headers_fill(),
so a non-NGX_OK return is no longer ignored.

Added a NULL check after JS_ToCStringLen() on the header key in
ngx_qjs_headers_ext_keys(), and replaced a stale "value" check with
"item" when testing JS_NewStringLen() for failure.