refactor av_delete

Some bad code generation/bad optimization from VC caused me to clean this
up. Dont write AvARRAY(av)[key] = NULL twice. It appears in 2 branches
just as written VC asm wise. Don't call sv_2mortal on a NULL SV*. It
works but less efficient. On VC2008 x64 -O1 this func dropped from 0x208
to 0x202 bytes of machine code after this patch.