BUG: integrate: odeint crashed when the integration time was 0.

When only a single time point (or only a single repeated time value)
was given along with the argument full_output=True, a KeyError was
raised because the _msgs dict didn't have the key 1 (the 'istate'
value returned by the Fortran solver when no work was done).

Closes gh-4282.
Merge pull request #4284 from WarrenWeckesser/bug-odeint-single-time

BUG: integrate: odeint crashed when the integration time was 0.