Description:
------------
Looking at the source code of how curl_multi_select() is implemented, there's various different ways of which values it can return.

And these includes:
- Number of descriptors from curl_multi_wait or curl_multi_fdset depending on the version
- curl version before 7.48 can set numfds to -1 in curl_multi_wait
- native call to select() can return -1 (without any curl error being set, so this return value highly depends on OS it seems)

Basically, many different little differences like that can produce wildly different results, and it's very hard to write a consistent library that uses multi curl.

Here's my library that implements multi curl: https://github.com/xPaw/Crimp.php

And after PHP 7.2.11 released, which uses curl_multi_wait, it looks like the current fix would be:
```
if( $Running ) {
	$Descriptors = curl_multi_select( $Master, 0.1 );

	if( $Descriptors === -1 && curl_multi_errno( $Master ) !== CURLM_OK ) {
		throw new \RuntimeException( 'curl_multi_select failed. error: ' . curl_multi_errno( $Master ) );
	}

	// count number of repeated zero numfds
	if( $Descriptors === 0 ) {
		if( ++$Repeats > 1 ) {
			usleep( 100 );
		}
	} else {
		$Repeats = 0;
	}
}
```

Note that I added a curl_multi_errno check if curl_multi_select returns -1.

I'm not entirely sure what's the elegant way of fixing this would be, but this is clearly a problem that keeps on reoccurring, for example in bug #74334