Fix some leaks that turned up while valgrinding the GVariant testcase.

These leaks are small and only occur when there is already an error in
the program: they are leaks of temp strings used when formatting a
critical message.

These show up as leaks again the testcase under the new "expect
messages" approach.  Previously, we fork()ed and these caused the
subprocess to abort, which is why this was not noticed before.