exception in fiber should make resume to raise in parent fiber context; close #1765

src/vm.c

@@ -1300,9 +1300,18 @@ mrb_context_run(mrb_state *mrb, struct RProc *proc, mrb_value self, unsigned int
           }
           else if (ci == mrb->c->cibase) {
             if (ci->ridx == 0) {
+              if (mrb->c == mrb->root_c) {
                 regs = mrb->c->stack = mrb->c->stbase;
                 goto L_STOP;
               }
+              else {
+                struct mrb_context *c = mrb->c;
+
+                mrb->c = c->prev;
+                c->prev = NULL;
+                goto L_RAISE;
+              }
+            }
             break;
           }
         }