Fixed possible SEGV in `Kernel#block_given?`; ref #3593

src/kernel.c

@@ -136,44 +136,38 @@ mrb_f_block_given_p_m(mrb_state *mrb, mrb_value self)
 {
   mrb_callinfo *ci = mrb->c->ci;
   mrb_value *bp;
-  mrb_bool given_p;
 
   bp = ci->stackent + 1;
   ci--;
   if (ci <= mrb->c->cibase) {
-    given_p = FALSE;
+    return mrb_false_value();
   }
-  else {
   /* block_given? called within block; check upper scope */
   if (ci->proc->env) {
     struct REnv *e = ci->proc->env;
-      mrb_value *sp;
 
     while (e->c) {
       e = (struct REnv*)e->c;
     }
-      sp = e->stack;
-      if (sp) {
-        /* top-level does not have block slot (alway false) */
-        if (sp == mrb->c->stbase)
+    /* top-level does not have block slot (always false) */
+    if (e->stack == mrb->c->stbase)
       return mrb_false_value();
-        if (e->cioff < 0) {
+    if (e->stack && e->cioff < 0) {
       /* use saved block arg position */
       bp = &e->stack[-e->cioff];
+      ci = 0;                 /* no callinfo available */
     }
     else {
       ci = mrb->c->cibase + e->cioff;
       bp = ci[1].stackent + 1;
     }
   }
-    }
-    if (ci->argc > 0) {
+  if (ci && ci->argc > 0) {
     bp += ci->argc;
   }
-    given_p = !mrb_nil_p(*bp);
-  }
-
-  return mrb_bool_value(given_p);
+  if (mrb_nil_p(*bp))
+    return mrb_false_value();
+  return mrb_true_value();
 }
 
 /* 15.3.1.3.7  */