AST dumping: Fix hAstNodeToStr() for VAR nodes with NULL symbol

(at least astCheckASSIGNToType() temporarily uses a VAR node like that)
Fix passing TYPEINIs/CALLCTORs to BYREF params

TYPEINIs and CALLCTORs represent temp vars, so they can be passed to
BYREF params just like VARs, DEREFs or IIFs. The old code didn't check for
TYPEINIs explicitly and ended up assuming that a node with dtype=STRING was
a CALL even though it could be a TYPEINI.