Since commit e1409ac5 (Support building shared libraries or modules
without soname, 2012-04-22) CMake crashes on the code

 add_library(foo SHARED foo.nolang)

because the logic to lookup the language's soname flag was moved from
cmTarget::GetLibraryNames to cmMakefile::GetSONameFlag without its check
for a NULL language.  Restore the check for NULL.

Add RunCMake.Languages test to cover language error cases like this one.