In `flisten` check that the stream is not in an error state, as well as not eof

This patch is inspired by the issue I described here: https://groups.google.com/d/msg/sage-devel/y79rSTdBLQA/LnkO66FCAwAJ

But in short, there are situations (such as a hangup) where reading/writing to stdio will result in an error. If, at this point, the LISP debugger is entered it tries to call (clear-input), which in turn calls flisten in a loop. However, it only checks for EOF, and not for errors.

This isn't a problem on Linux (it seems), because getc sets the EOF indicator on the stream. However, this is not a requirement, and on some platforms it seems (particularly Cygwin, which uses the newlib C library) only set the error indicator but not EOF. This results in an infinite loop. The simplest solution seems to be to just check ferror here as well.