Fix Mockery::fetchMock() without initialized container (1.3 branch)

[PhrozenByte]

See #1113

Calling Mockery::fetchMock() first doesn't make much sense and likely indicates a logic error, but Mockery should nevertheless throw Call to a member function fetchMock() on null errors.

Maybe we should throw a LogicException instead when no container was initialized yet (just like Mockery::self())? If no container was initialized yet, Mockery::fetchMock() will always return null, just like if the mock in question simply doesn't exist (yet). Other opinions?

[davedevelopment]

Thanks!