Skip memcpy(x, x) in qunique().

It has undefined behavior.  Follow the precedent of commit
9a9473f3cc.  No back-patch, since the
master branch alone has this function.

Discussion: https://postgr.es/m/20191229070221.GA13873@gust.leadboat.com

src/include/lib/qunique.h

@@ -30,8 +30,9 @@ qunique(void *array, size_t elements, size_t width,
 
 	for (i = 1, j = 0; i < elements; ++i)
 	{
-		if (compare(bytes + i * width, bytes + j * width) != 0)
-			memcpy(bytes + ++j * width, bytes + i * width, width);
+		if (compare(bytes + i * width, bytes + j * width) != 0 &&
+			++j != i)
+			memcpy(bytes + j * width, bytes + i * width, width);
 	}
 
 	return j + 1;
@@ -55,8 +56,9 @@ qunique_arg(void *array, size_t elements, size_t width,
 
 	for (i = 1, j = 0; i < elements; ++i)
 	{
-		if (compare(bytes + i * width, bytes + j * width, arg) != 0)
-			memcpy(bytes + ++j * width, bytes + i * width, width);
+		if (compare(bytes + i * width, bytes + j * width, arg) != 0 &&
+			++j != i)
+			memcpy(bytes + j * width, bytes + i * width, width);
 	}
 
 	return j + 1;