Fix exception when path not found.

Add a default return value in a call in the GetLibraryByBuildId()
function to avoid an exception if a file cannot be found in the
symbol directory.

Bug: 216232380

Test: Ran the input from the bug. Verified without the change, the
Test: exception occurs and with the fix, no exception.
Test: Ran unit tests of stack_core.py.
Change-Id: I015982eb835f5a3d7f03dcc3e0b1627b0c1a7cfe
1 file changed