Fix handling of zero-byte chunk from hyper.

[jsha]

When curl is built with Hyper, nghttp2, and most TLS backends (other than crustls), it can produce reads from uninitialized memory. This is due to Hyper providing body chunks of length 0, and hyper_body_chunk passing those to Curl_client_write. When Curl_client_write gets a buf with a len of 0, it calls strlen on buf and uses that as the len:

curl/lib/sendf.c

Lines 604 to 620 in f014eec

	/* Curl_client_write() sends data to the write callback(s)
	The bit pattern defines to what "streams" to write to. Body and/or header.
	The defines are in sendf.h of course.
	If CURL_DO_LINEEND_CONV is enabled, data is converted IN PLACE to the
	local character encoding. This is a problem and should be changed in
	the future to leave the original data alone.
	*/
	CURLcode Curl_client_write(struct Curl_easy *data,
	int type,
	char *ptr,
	size_t len)
	{
	struct connectdata *conn = data->conn;
	if(0 == len)
	len = strlen(ptr);

.

Fixes hyperium/hyper#2512.

Thanks to @kevinburke for reporting.

[kevinburke]

I built this patch locally and confirmed it fixed the issue I reported in hyperium/hyper#2512.

[bagder]

thanks!