Check get_gwp_asan_callbacks before calling.

When using the bootstrap linker, the get_gwp_asan_callbacks is
not set. Therefore, check it is not nullptr before calling it
during crash processing.

Bug: 284098779

Test: Ran crasher64 using /system/bin/bootstrap/linker64 and verify
Test: debuggerd code does not crash.
Test: All unit tests pass.
Change-Id: Ifc710fe4bef24661700444a1b69432bfc29d580f
Merged-In: Ifc710fe4bef24661700444a1b69432bfc29d580f
(cherry picked from commit 004a1673)