reactor: guard against a potential segfault

If this `if/else if` falls through, we'll dereference event.data.ptr
below.

Caught by the static analyzer:
system/bt/gd/os/linux_generic/reactor.cc:131:37: warning: Forming
reference to null pointer [clang-analyzer-core.NonNullParamChecker]

Bug: None
Test: TreeHugger
Change-Id: I5800ec3772db118ae6c75b40c638fea480248db6