Clean up OatHeader::GetStoreValueKey*().

Replace helper function ParseString() with calls to memchr()
and make the OatHeader::GetStoreValueKey*() more readable.
This also avoids using a pointer to `end + 1` when we fail
to find the end of the value (this was benign as that `+1`
should not cause arithmetic overflow anyway).

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Change-Id: I2fa9043f42048689a64fba3ab1fc3845f4684544
1 file changed