Added return in Version::Get::State::Match to quiet warning.

Added unreached return at the end of Version::Get::State::Match
to stop this _incorrect_ warning:

    version_set.cc:376:5: warning: control reaches end of
    non-void function [-Wreturn-type]

This warning was being emitted when building with clang 6.0.1-10
and also emitted by lgtm.com when statically analyzing leveldb even
though all SaverState enumeration values were handled.

PiperOrigin-RevId: 272455474

db/version_set.cc

@@ -373,6 +373,10 @@ Status Version::Get(const ReadOptions& options, const LookupKey& k,
           state->found = true;
           return false;
       }
+
+      // Not reached. Added to avoid false compilation warnings of
+      // "control reaches end of non-void function".
+      return false;
     }
   };