Fix handleFromPtr missing Reference in Maps

[EvanOman]

This patch fixes a panic I was encountering when using a map whose value was a non-value-type like another Map, Struct, Slice, etc.

Related:

• Can't return a slice of slices ([][]bool) #346
• Panic when function returns a map of non-pointer structs #329
• Commit by @richecr that fixed a similar issue for slices

This fix makes gopy useable for me but I have not added new tests (yet)

[rcoreilly]

Thanks for the fix and review. Looks sensible to me.