[SPARK-17618] Guard against invalid comparisons between UnsafeRow and other formats

This patch ports changes from #15185 to Spark 2.x. In that patch, a  correctness bug in Spark 1.6.x which was caused by an invalid `equals()` comparison between an `UnsafeRow` and another row of a different format. Spark 2.x is not affected by that specific correctness bug but it can still reap the error-prevention benefits of that patch's changes, which modify  ``UnsafeRow.equals()` to throw an IllegalArgumentException if it is called with an object that is not an `UnsafeRow`.

Author: Josh Rosen <joshrosen@databricks.com>

Closes #15265 from JoshRosen/SPARK-17618-master.

sql/catalyst/src/main/java/org/apache/spark/sql/catalyst/expressions/UnsafeRow.java

@@ -31,6 +31,7 @@ import com.esotericsoftware.kryo.KryoSerializable;
 import com.esotericsoftware.kryo.io.Input;
 import com.esotericsoftware.kryo.io.Output;
 
+import org.apache.spark.sql.catalyst.InternalRow;
 import org.apache.spark.sql.types.*;
 import org.apache.spark.unsafe.Platform;
 import org.apache.spark.unsafe.array.ByteArrayMethods;
@@ -577,8 +578,12 @@ public final class UnsafeRow extends MutableRow implements Externalizable, KryoS
       return (sizeInBytes == o.sizeInBytes) &&
         ByteArrayMethods.arrayEquals(baseObject, baseOffset, o.baseObject, o.baseOffset,
           sizeInBytes);
+    } else if (!(other instanceof InternalRow)) {
+      return false;
+    } else {
+      throw new IllegalArgumentException(
+        "Cannot compare UnsafeRow to " + other.getClass().getName());
     }
-    return false;
   }
 
   /**