Don't allow bad lines of output, as seen in the rdf property, to
crash the whole show. Make sure there is a : present. Example of
output that crashes when identify outputs lines containing whitespace only:

(I have added | marks to show where the whitespace ends. The | marks are
not in the actual output of identify.)

    rdf:Alt: |
     |
    |
    signature: a42e682fbf3b7a4aa29577935f6deed66e7a3e688a3edb8e3a0ccc7aad21d

My patch is to ignore lines with no : at all in them. Otherwise all
properties encountered before this point, including the format property,
are lost.