BUG/MINOR: stats: use strncmp() instead of memcmp() on health states

BUG/MINOR: stats: use strncmp() instead of memcmp() on health states

The reports for health states are checked using memcmp() in order to
only focus on the first word and possibly ignore trailing %d/%d etc.
This makes gcc unhappy about a potential use of "" as the string, which
never happens since the string is always set. This resulted in commit
c4e6460f6 ("MINOR: build: Disable -Wstringop-overflow.") to silence
these messages. However some lengths are incorrect (though cannot cause
trouble), and in the end strncmp() is just safer and cleaner.

This can be backported to all stable branches as it will shut a warning
with gcc 8 and above.