If an example fails, the clipboard may reflect old values

**Describe the bug**
A clear and concise description of what the bug is followed by the steps to reproduce it:
1. Create three examples:
```python
>>> print("val = A")
val =
>>> print("vaxxx = B")
val =
>>> print("") # byexample: +paste
B
```
2. Then run it as usual

**Expected behavior**
The third example should fail saying that is not in the clipboard because the previous example tried to capture it but it failed.

Instead, because the same was captured before the third example gets executed and it fails saying that "A" is not the expected "B" which it may confuse more or worst, it may lead to false positives.

**byexample version**
byexample 8.1.0 (Python 3.5.3)