Fix compile error with Clang related to diagnostic pragma

Fix
```
pragma diagnostic pop could not pop, no matching push
```

The GCC documentation says "If a pop has no matching push, the
command-line options are restored." but that's apparently not working here.

misc/levenshtein.cpp

@@ -33,6 +33,7 @@ void initDistanceArray(DistanceArray &distanceArray, const size_t size1, const s
     const auto maxDistance(size1 + size2);
     // ignore warning about null pointer dereference for now (which is *likely* not correct)
 #ifdef __GNUC__
+#pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Wnull-dereference"
 #endif
     distanceArray.at(0, 0) = maxDistance;