On first close, we set the file pointer to 0 to indicate that it
is not valid anymore. As PrismLog implements Close/AutoCloseable, multiple
close calls have to be supported.
Before, calling close multiple times could result in double free
crashes on Linux (seen in development branches, not master).
We also check that we don't write on a closed FILE pointer, but raise
an exception instead. Here, we use a RuntimeException
(IllegalStateException) instead of a PrismException as changing the
PrismLog interface would be quite disruptive to the rest of the code.
Joachim Klein
c61ca02d99