Be more robust against premature process crash.

In case a process crashes right after calling `attachApplication`, it is
possible that binder wasn't able to get the PID of the process while
delivering the transaction, resulting in a PID of 0. libprocessgroup's
killProcessGroup() requires a positive PID, so only call when we got a
valid PID.

Bug: 297833427
Test: N/A
Change-Id: I7f815bcccfac05cd84f53a1dd9d5b457275b7558