Fix inc/dec of undef var with error handler

Set the variable to null after emitting the undef var notice
rather than before. This avoids an assertion failure if the var
is unset by t

Fix inc/dec of undef var with error handler

Set the variable to null after emitting the undef var notice
rather than before. This avoids an assertion failure if the var
is unset by the error handler.

The flip side is that this may cause a leak instead, but that's
the more harmless outcome.

Fixes oss-fuzz #36604.

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