Fix gcc strict aliasing warnings with function pointers too.

The commit 40bf877 fixed warnings about converting between function and object
pointers but introduced warnings about breaking strict-aliasing rules which
appear with -Wstrict-aliasing which is implicitly enabled by -O2.

Avoid these warnings as well by using an intermediate union for conversion
instead of casts trickery.
Merge pull request #129 from vadz/warn-fix

Fix gcc strict aliasing warnings with function pointers too.