Clear local (but non-argument) variables in OP_ENTER.

Otherwise, the following script prints an uninitialized value.

def f(*a)
  if false
    b = 15
  end
  p b
end
f(1,2,3)

src/vm.c

@@ -1637,6 +1637,10 @@ RETRY_TRY_BLOCK:
         }
         pc += o + 1;
       }
+      /* clear local (but non-argument) variables */
+      if (irep->nlocals-len-2 > 0) {
+        stack_clear(&regs[len+2], irep->nlocals-len-2);
+      }
       JUMP;
     }