Fix unintialized value bug in gzputc() introduced by const patches.
Avoid the use of an uninitialized value when the write buffers have
not been initialized. A recent change to avoid the use of strm->
next_in in order to resolve some const conflicts added the use of
state->in in its place. This patch avoids the use of state->in
when it is not initialized. Nothing bad would actually happen,
since two variables set to the same unintialized value are
subtracted. However valgrind was rightly complaining. So this
fixes that.