tinyscheme: Fix an uninitialized value read, EOF_OBJ

Maybe send this upstream.

==26827== Conditional jump or move depends on uninitialised value(s)
==26827==    at 0x406DD7: opexe_0 (scheme.c:2577)
==26827==    by 0x40E2C2: Eval_Cycle (scheme.c:4467)
==26827==    by 0x40F090: scheme_load_named_file (scheme.c:4826)
==26827==    by 0x40F63E: main (scheme.c:5021)