scripts/mupdfwrap.py: fixed handling of non-copyable wrapper classes.
scripts/mupdfwrap.py: fixed handling of non-copyable wrapper classes.

class ClassExtras:get(): insert empty ClassExtra() into self.items if we are
asked for non-existent item. Otherwise our caller's modifications will be lost
if someone calls get() again.

This results in detection of a few non-copyable wrapper classes, for example:

    fz_compressed_buffer
    fz_layout_block
    pdf_alert_event
    ...

ArtifexSoftware/mupdf,ArtifexSoftware/mupdf,ArtifexSoftware/mupdf,TamirEvan/mupdf,ArtifexSoftware/mupdf,TamirEvan/mupdf,ArtifexSoftware/mupdf,TamirEvan/mupdf,ArtifexSoftware/mupdf,ArtifexSoftware/mupdf,TamirEvan/mupdf,TamirEvan/mupdf,TamirEvan/mupdf,ArtifexSoftware/mupdf,TamirEvan/mupdf,TamirEvan/mupdf