libweston: do not forget to check if test_data is NULL

Since commit "libweston: add required_capabilities test suite quirk" a new function that depends on test_data is being called in wet_main().

We should check if test_data is NULL before calling it, otherwise we have a segfault when running outside the test suite.

Signed-off-by: Leandro Ribeiro leandro.ribeiro@collabora.com