Remove 'bogus' error condition in str_pad()

[Girgias]

As per @nikic request in #4601 (comment)

I'm not entirely sure we should drop it but more fix it in a way that memory allocation doesn't fail. Or is the fatal error alright ? Because even with the current check a fatal error is possible (see variation5 test)

[nikic]

Fatal error is correct here. You'll have to adjust the test expectation for debug builds though (you can make liberal use of %s, we're only interested in seeing some form of OOM error, not the specific message).

[Girgias]

Fatal error is correct here. You'll have to adjust the test expectation for debug builds though (you can make liberal use of %s, we're only interested in seeing some form of OOM error, not the specific message).

Same should be done for chunk_split then I imagine.