RB_GC_GUARD() is called on unintialized variables in BigDecimal_DoDivmod()

[eregon]

These 3 calls:

bigdecimal/ext/bigdecimal/bigdecimal.c

Lines 1742 to 1744 in 4f93954

	RB_GC_GUARD(dv.bigdecimal);
	RB_GC_GUARD(md.bigdecimal);
	RB_GC_GUARD(res.bigdecimal);

call RB_GC_GUARD() on variables which might be uninitialized and so contain garbage values.

This seems to apparently work fine on CRuby, but reading the docs I am not sure it is a correct usage.
It seems unusual to do this.

On TruffleRuby this caused transient errors: truffleruby/truffleruby#4274
Probably this is something we can improve in TruffleRuby, OTOH this is I think the first extension we notice doing this.

Given the code is just:

bigdecimal/ext/bigdecimal/bigdecimal.c

Lines 1739 to 1745 in 4f93954

	Done:
	RB_GC_GUARD(a.bigdecimal);
	RB_GC_GUARD(b.bigdecimal);
	RB_GC_GUARD(dv.bigdecimal);
	RB_GC_GUARD(md.bigdecimal);
	RB_GC_GUARD(res.bigdecimal);
	return true;

One fix would be to replace goto Done; with the guard + return true, but omit the guards for unitialized variables at that point.
Or, the variables could be always initialized.