Bug 698592: Mark variable fz_var(), avoiding optimization.
Bug 698592: Mark variable fz_var(), avoiding optimization.

The change in 2707fa9e8e6d17d794330e719dec1b08161fb045
in build_filter_chain() allows for the variable chain
to reside in a register, which means that the bug is
likely to only be visible if built under optimization.

First the chain variable is transferred to chain2, then
set to NULL, then when an exception occurs in build_filter()
the filter chain will be freed by build_filter(). Next
the expectation is that execution proceeds to fz_catch()
where fz_drop_stream() would be called with chain == NULL.

However due to the chain variable residing in a register,
its value is not NULL as expected, but was reset to its
original value upon the exception (since they use setjmp()),
hence fz_drop_stream() is called with a non-NULL value.

Marking the chain variable with fz_var() prevents the
compiler from allowing the chain variable to reside in
a register and hence its value will remain NULL and
never be reset.

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