text: turn the mark off when justifying, to not confuse an undo

When leaving the mark on while justifying and then undoing the
justification, things are likely to get messed up.  My applying
David's patches only partially caused this breakage.

This fixes https://savannah.gnu.org/bugs/?55074.

src/text.c

@@ -2243,6 +2243,9 @@ void do_justify(bool full_justify)
 		return;
 	}
 
+	/* We cannot (yet) justify a marked region, so turn the mark off. */
+	openfile->mark = NULL;
+
 	/* Prepare to put the text we want to justify in the cutbuffer. */
 	cutbuffer = NULL;
 	cutbottom = NULL;