[patch, fortran] Factorize symbol refcounting.

[Mikael Morin]

Hello,

this takes the symbol reference count management code from free_sym_tree 
into its own function so that it can be shared.

I have made the gfc_internal_error (in case reference count becomes 
negative) a gcc_assert as the error message was not very 
informative/useful from a user point of view.

OK for trunk when regression test finishes ?

Mikael
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