Do not allow lines of identify output containing whitespace only to crash the whole show

Do not allow lines of identify output containing whitespace only 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, and code in the imagemagick module that assumes there is a format property crashes.

I have attached the JPEG image that caused this result although I don't know whether github will regenerate it and lose the RDF property. I've seen this crash before, and now I know it relates to parseIdentify becoming confused, so if I spot other examples I'll track those down and patch them as well.

![acfhorizontallogo](https://f.cloud.github.com/assets/32125/335305/d644f2c0-9c88-11e2-936a-ece5bb39fb68.jpg)