hw/block/pflash_cfi01: Don't decrement pfl->counter below 0

hw/block/pflash_cfi01: Don't decrement pfl->counter below 0

In pflash_write() Coverity points out that we can decrement the
unsigned pfl->counter below zero, which makes it wrap around.  In
fact this is harmless, because if pfl->counter is 0 at this point we
also increment pfl->wcycle to 3, and the wcycle == 3 handling doesn't
look at counter; the only way back into code which looks at the
counter value is via wcycle == 1, which will reinitialize the counter.
But it's arguably a little clearer to break early in the "counter ==
0" if(), to avoid the decrement-below-zero.

Resolves: Coverity CID 1547611
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Message-ID: <20240731143617.3391947-4-peter.maydell@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>