avoid an assert in parse_fonts

This is an assertion of the type that catches an error (in user-supplied
data) that should be handled, namely the case where the /Font entry
of a dictionary is expected to be itself a dictionary but isn't.
The code already contains a path for the case where the /Font entry
is missing (return false) and I suppose the same, including the
TODO item "figure out how to handle", might as well apply instead
of the assertion.

Fixes #45.