ImageMagick module hangs file upload process when convert of PDF file results in more than 64K chars of warnings

This is the same issue as in the 7.x branch, the code used to run imagemagick is basically the same. This is a port of the patch I made for the 7.x branch.

Original Comment:

There is a race condition where the stderr pipe can fill up while we are reading the stdout pipe. This causes imagemagic to stop processing and wait for the pipe to empty, since we continue to read the stdout pipe until it closes, we end up in a deadlock with php waiting for the application to close stdout and the application is waiting for php to read the data from stderr.

Here is the patch updated to 8.x-2.x, the code is again basically the same, just in a different place.

Testing this is tricky. I think it needs more testing-fu then I have. Maybe something like shipping with a bad PDF in the test folder then running a convert on it. Not sure exactly. I'm fairly convinced it will depend on the system installed versions of IM, GS and the libopenjp2, because that is what determines how much output goes to stdout.