visitReturn fixes

Return statements were only checked to be inside a definition if they
had an explicit return value.

The test also included class definitions, so class Foo: return 7 got
no warning.

The implicit None return type was ignored if the same function
contained another return statement with an explicit value.

Fixes new tests:

• misplaced_return_plain
• misplaced_return_class
• funccall_maybe_def_return

No test regressions.