Running a query containing the `REGEXP` function with an invalid pattern breaks

[se7entyse7en]

If the query contains the REGEXP function and the pattern is invalid, then the server stops working and doesn't recover anymore. At the first run it answers:

From the second run and on it answers:

From the log it seems that the bug is caused by how the wrong pattern is handled in gitbase. Here's the log:

[smacker]

it's a panic in gitbase. You better open an issue in https://github.com/src-d/gitbase
Thanks!

[carlosms]

Closing since this is already tracked in https://github.com/src-d/gitbase/issues/682