zero-fill before GC mark

Depending on architectures, setjmp might not fully fill a jmp_buf.
On such machines the union can contain wobbly bits. They are then
scanned during mark_locations_array().  This is bad.

gc.c

@@ -4598,6 +4598,7 @@ mark_current_machine_context(rb_objspace_t *objspace, rb_execution_context_t *ec
     VALUE *stack_start, *stack_end;
 
     FLUSH_REGISTER_WINDOWS;
+    memset(&save_regs_gc_mark, 0, sizeof(save_regs_gc_mark));
     /* This assumes that all registers are saved into the jmp_buf (and stack) */
     rb_setjmp(save_regs_gc_mark.j);