The VCC_GlobalSymbol() function might be called twice for the same
symbol. For example a subroutine symbol may be created when the sub
keyword is first encountered, but it was referenced by a call action
before the subroutine definition.

The main problem the leak is causing is lsan's output polluting test
cases looking at the screen output of varnishd, making the lines we
care about scroll out to oblivion. To remedy this, VCC_GlobalSymbol()
idempotence becomes free of side effects.