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 (closed).