Fix browser body class issue (on Chrome iOS but potentially others)

In 'thematic/library/extensions/dynamic-classes.php' in the
thematic_browser_class_names() function starting at line 366, when
$matches[1] gets called, there isn't always an index of '1', so the
array key needs to be checked with isset() before being used for every
$matches[1] call in that function.

This should be fixed now.

Issue: https://github.com/ThematicTheme/Thematic/issues/105