Fix userData handling in compilerCallback

Although userData is an unsafe.Pointer, it is internally used as key
to access data in a map, rather than being dereferenced. So the nil
check made no sense.

Close #8

Add yara3.3 build tag that can be used to eliminate features introduc…

…ed in YARA 3.4

This removes the need for a separate (and outdated) compat-yara-3.3 branch.