tools/cgsnapshot: add ret value fix in parse_controllers()

tools/cgsnapshot: add ret value fix in parse_controllers()

Fix the unused ret value warning, reported by Coverity:

CID 258275 (#1 of 1): Unused value (UNUSED_VALUE)returned_value:
Assigning value from display_controller_data(controllers, program_name)
to ret here, but that stored value is overwritten before it can be used.

The parse_controllers(), doesn't check for errors in the value returned
by display_controller_data(). The return value might very well contain
an error, that might go unnoticed. Fix it by adding a check for the
return value.

Signed-off-by: Kamalesh Babulal <kamalesh.babulal@oracle.com>
Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>