debuggerd: Ignore SIGPIPE

The system server may prematurely close the connection to
/data/system/ndebugsocket if it's not interested in the data
from debuggerd.  If it does so, we don't want to die due to a
SIGPIPE.

Change-Id: Iaef1f497bcd630144e6df6a06644a3293b85b6e0