Fix SF bug #505315: Make free and cell vars show up consistently in locals().

PyFrame_FastToLocals() and PyFrame_LocalsToFast() had a return if
f_nlocals was 0.  I think this was a holdover from the pre 2.1 days
when regular locals were the only kind of local variables.

The change makes it possible to use a free variable in eval or exec if
it the variable is also used elsewhere in the same block, which is
what the documentation says.