fix small issue in sql.py which cause AttributeError

[io614]

When debug mode is off, sometimes a

AttributeError: '_AppCtxGlobals' object has no attribute '_connection' 

is thrown during operation. This is solved by checking that _connection exists in the flask application context before looking up its value.

[dmalan]

Thanks! Turns out you uncovered some other latent bugs that we're going to fix a bit more thoroughly in #138!