Silence compiler warning

Silence compiler warnings about methods with return value and potentially not returning anything.
This happens with gcc8 and the `-O2 -Wall -D_FORTIFY_SOURCE=2 -fstack-protector-strong -funwind-tables -fasynchronous-unwind-tables -fstack-clash-protection -std=c++11` flags passed to it by default on openSUSE.

src/modules/data/HashFunction.cpp

@@ -608,6 +608,7 @@ HashFunction *HashFunction::getHashFunction(Function function)
 		return nullptr;
 	// No default for compiler warnings
 	}
+    return nullptr;
 }
 
 bool HashFunction::getConstant(const char *in, Function &out)