You are in a twisty little maze of code paths, all different. Get rid

You are in a twisty little maze of code paths, all different.  Get rid
of the if (tree) tests in dissect_mac_header_generic_decoder(), as some
of the stuff done inside there should be done even if we're not building
a protocol tree, e.g. CRC stuff and calling subdissectors.

Get rid of a gratuitous pair of curly braces, and fix the indentation of
one line, while we're at it.

svn path=/trunk/; revision=52208