Instead of breaking out of the switch-in-while construct using `return` this
uses `goto out` which makes it possible to share a lot of the out-variable
assignment code in all the cases.

I also replaced the nasty `while(true)` business by the real loop
condition: `while(*c == NULL)`. All `break` calls inside the switch aready
have either a check for NULL or an assignment of `c` to NULL so this should
not change any behaviour.

Using `goto out` also allowed me to remove another minor wart: In the
MVAR_*/WEAK cases the popOff() call used to happen before reading the
stackElement. This looked like a use-after-free hazard to me as the stack
is allocated in blocks and depletion of a block could mean it getting freed
and possibly overwritten by zero or garbage, depending on the block
allocator's behaviour.