Fix bug #68532: convert.base64-encode omits padding bytes

[mcq8]

The memory stream will now read as much as possible and only set the EOF when you try to read when there is nothing left.
This is the way file streams do it and apparently some filter algorithms as convert.base64-encode depend on it.

[mcq8]

Can someone with write access rerun travis CI?
It is just a random failure because a socket port was in use during the tests.

[jpauli]

This has been merged against 5.5 and up

[jails]

Before this bugfix, feof() returned true one byte earlier as it should. However this bugfix introduced a glitch since EOF should be marked as true when the read call is done with a length > stream length right away. With this bugfix, read() needs to be called twice: https://bugs.php.net/bug.php?id=68948.