exec.c: fix setting 1-byte-long watchpoints

With commit 05068c0dfb5b 'exec.c: Relax restrictions on watchpoint length
and alignment' it's no longer possible to set 1-byte-long watchpoint
because of incorrect address range check.
Fix that by changing condition that checks for address wraparound.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1411016616-29879-1-git-send-email-jcmvbkbc@gmail.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>