PGMonitor: fix uninitialized scalar variable

Fix type handling in dump_stuck_pg_stats. If type is type doesn't
match to known PGMap::STUCK_* type print out a message and return
directly from function.

CID 1030132 (#2 of 2): Uninitialized scalar variable (UNINIT)
 uninit_use_in_call: Using uninitialized value "stuck_type" when calling
 "PGMap::dump_stuck(ceph::Formatter *, PGMap::StuckPG, utime_t) const"

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