libradosstriper/striping.cc: fix strdup memory leaks

Fix memory leak caused by using std::string to hold result
of strdup call returned from getObjName().

Fix for Coverity issues:

CID 1221525 (#1 of 1): Resource leak (RESOURCE_LEAK)
 leaked_storage: Failing to save or free storage allocated by
 this->getObjName(soid, 0UL) leaks it.

CID 1221526 (1-3 of 3): Resource leak (RESOURCE_LEAK)
 leaked_storage: Failing to save or free storage allocated by
 this->getObjName(soid, *) leaks it.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>