Check for valid column titles (non-zero-length) and valid column formats

Check for valid column titles (non-zero-length) and valid column formats
in preference settings.

In the process of doing that, fix a memory leak (we were handing a null
pointer, rather than a pointer to the list of strings in
"column.format", to "clear_string_list()").

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@3775 f5534014-38df-0310-8fa8-9805f1628bb7