Fix crash when checking symbol encoding

	[Bug #20245]

	We sometimes pass in a fake string to sym_check_asciionly. This can crash
	if sym_check_asciionly raises because it creates a CFP with the fake
	string as the receiver which will crash if GC tries to mark the CFP.

	For example, the following script crashes:

	    GC.stress = true
	    Object.const_defined?("\xC3")