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".
Merge pull request #3139 from garbear/seeing-double

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