Fix direct leak

closes https://github.com/assimp/assimp/issues/3747

code/AssetLib/HMP/HMPLoader.cpp

@@ -158,6 +158,9 @@ void HMPImporter::InternReadFile(const std::string &pFile,
         szBuffer[3] = ((char *)&iMagic)[3];
         szBuffer[4] = '\0';
     
+        delete[] mBuffer;
+        mBuffer = nullptr;
+        
         // We're definitely unable to load this file
         throw DeadlyImportError("Unknown HMP subformat ", pFile,
                                 ". Magic word (", szBuffer, ") is not known");