Do a null check on the OatQuickMethodHeader.

Even if we found a method at the bottom of the stack, we may be
executing something unrelated to it. That's when GetOatQuickMethodHeader
will return null. So do a null check and return if we did not find any
compiled code.

Test: build
Bug: 173753901
Change-Id: I05595d601e0523b1254581c306b157b273828c53
1 file changed