[BUGFIX] Handle post_max_size = 0

Although the PHP docs do not mention it, the default configuration
files in the repository explain that post_max_size = 0 means
"unlimited" size.

Adjust the checks in the code to adhere to that.

Releases: master, 6.2
Resolves: #66208
Change-Id: I256512fea5ef71fa570ad4fe925145c403c80989
Reviewed-on: http://review.typo3.org/38436


Reviewed-by: Stephan Großberndt <stephan@grossberndt.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Xavier Perseguers <xavier@typo3.org>
Tested-by: Stephan Großberndt <stephan@grossberndt.de>
Tested-by: Xavier Perseguers <xavier@typo3.org>