VECTOR: Passing parameters with side effects to macros is dangerous.

* Fix several instances where we were bumping a ref in the parameter and
then unrefing the object if it failed.  The way the AST_VECTOR_APPEND()
and AST_VECTOR_REPLACE() macros are implemented means if it fails the new
value was never evaluated.

Change-Id: I2847872a455b11ea7e5b7ce697c0a455a1d0ac9a