Add a check for NULL nlr_top in nlr_jump.

If no nlr_buf has been pushed, and an nlr_jump is called, then control
is transferred to nlr_jump_fail (which should bail out with a fatal
error).

unix/main.c

@@ -293,6 +293,7 @@ void pre_process_options(int argc, char **argv) {
 }
 
 int main(int argc, char **argv) {
+    nlr_jump(0);
     volatile int stack_dummy;
     stack_top = (void*)&stack_dummy;
 
@@ -447,3 +448,8 @@ int DEBUG_printf(const char *fmt, ...) {
     va_end(ap);
     return ret;
 }
+
+void nlr_jump_fail(void *val) {
+    printf("FATAL: uncaught NLR %p\n", val);
+    exit(1);
+}