abrt-watch-log: handle a case when child doesn't process its input

There is a suspicion that in some cases we generate many (thousands)
Xorg problem dirs from one crash. See rhbz#877464.

Also users report:
"Sometimes I'd go a week or two with
no problem then I'd get multiple crashes in X which would clog up
abrtd and seemingly hang the system. I'd find 10s of thousands of
xorg crash captures in /var/spool/abrtd at times."

This change closes one code path which may be responsible.

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>