moving: the current chunk cannot be beyond the last chunk of a line

When determining the leftedge of the current chunk, it is not simply
the leftedge that corresponds to the placewewant, but the leftedge that
corresponds to the minimum of the placewewant and the full line span.

This fixes https://savannah.gnu.org/bugs/?50653.

src/move.c

@@ -463,7 +463,12 @@ void do_up(bool scroll_only)
 
 #ifndef NANO_TINY
     if (ISSET(SOFTWRAP)) {
-	leftedge = (openfile->placewewant / editwincols) * editwincols;
+	size_t realspan = strlenpt(openfile->current->data);
+
+	if (openfile->placewewant < realspan)
+	    realspan = openfile->placewewant;
+
+	leftedge = (realspan / editwincols) * editwincols;
 	target_column = openfile->placewewant % editwincols;
     }
 #endif
@@ -514,7 +519,12 @@ void do_down(bool scroll_only)
 
 #ifndef NANO_TINY
     if (ISSET(SOFTWRAP)) {
-	leftedge = (openfile->placewewant / editwincols) * editwincols;
+	size_t realspan = strlenpt(openfile->current->data);
+
+	if (openfile->placewewant < realspan)
+	    realspan = openfile->placewewant;
+
+	leftedge = (realspan / editwincols) * editwincols;
 	target_column = openfile->placewewant % editwincols;
     }
 #endif