[android] Fixed missing error message in debug log, "Unable to load '.so file', reason: (null)"

Error message comes from dlerror() in xbmc/cores/DllLoader/SoLoader.cpp when CAndroidDyload::Open() fails. However, dlerror() is also called inside CAndroidDyload::Open(). This causes the second, outer dlerror() to return NULL, because (from the documentation) "dlerror() returns NULL if no errors have occurred since initialization or since it was last called".