httpc: fix assertion fail after curl write error

After executing curl request we need to process curl_request code. It
might be CURLE_WRITE_ERROR. We had special case for it, which assumed
diagnostic message being set and contained corresponding assert, though
it is incorrect. Better way is to handle it as any other non-standard
event.

It was discovered while adding accept_encoding option. In case of
unknown encoding curl_request code is currently set to CURLE_WRITE_ERROR
and therefore we come to an assert assuming we have some diagnostics
set. However, it is not being set and it is totally fine. This means we
are failing on assert and it is not correct behavior.

Prerequisites: #4232

Reviewed-by: Alexander Turenko <alexander.turenko@tarantool.org>

src/httpc.c

@@ -429,16 +429,12 @@ httpc_execute(struct httpc_request *req, double timeout)
 	case CURLE_COULDNT_RESOLVE_PROXY:
 	case CURLE_COULDNT_RESOLVE_HOST:
 	case CURLE_COULDNT_CONNECT:
+	case CURLE_WRITE_ERROR:
 		/* 595 Connection Problem (AnyEvent non-standard) */
 		req->status = 595;
 		req->reason = curl_easy_strerror(req->curl_request.code);
 		++env->stat.failed_requests;
 		break;
-	case CURLE_WRITE_ERROR:
-		/* Diag is already set by curl_write_cb() */
-		assert(!diag_is_empty(&fiber()->diag));
-		++env->stat.failed_requests;
-		return -1;
 	case CURLE_OUT_OF_MEMORY:
 		diag_set(OutOfMemory, 0, "curl", "internal");
 		++env->stat.failed_requests;