av_exists: dont make a mortal never to use it

av_exists: dont make a mortal never to use it

Make av_exists slightly smaller and faster by reducing the liveness of a
mortal SV and using a NN SvTRUE_nomg.

There were 3 cases, where this mortal would be created, yet a return
happened and the mortal went unused and was wasted. So move the mortal
creation point closer to where it is first used. Also var sv will never be
null, so use a NN version of SvTRUE_nomg created in commit [perl #115870].
The retbool line isn't actually required for optimization reasons, but was
created just in case something in the future changes or some unknown
compiler does something inefficiently.

For me with 32 bit x86 VC 2003, before av_exists was 0x1C2, after 0x1B8.

Comment from committer: Includes SvTRUE_nomg_NN from [perl #115870].