samples: drivers: current sensing : Exit from testcase if device not found

Exiting from the test case when the i2c device is not found, this
to avoid a null pointer dereference

This issue was reported by Coverity

Coverity-CID: 151982

Change-Id: Ib11224ef2a78170f6e4d20545f645b4f2ea4a181
Signed-off-by: Sergio Rodriguez <sergio.sf.rodriguez@intel.com>