justify: set `x = 0` for the undo item, for when using --cutfromcursor

When --cutfromcursor is active, 'current_x' needs to be set to zero when
doing a justification, so that the correct starting position gets stored
in the undo item.  (Without --cutfromcursor, the value of 'current_x'
does not matter.)

This fixes https://savannah.gnu.org/bugs/?65317.
The issue was indirectly reported by `correctmost`.

Bug existed in this form since version 5.0, commit ae5a4ece.

Between versions 4.0 and 5.0, nano would not eat a line but would
instead crash when undoing a justification that was done with the
cursor away from the left edge.

src/text.c

@@ -1871,7 +1871,8 @@ void justify_text(bool whole_buffer)
 #endif
 			refresh_needed = TRUE;
 			return;
-		}
+		} else
+			openfile->current_x = 0;
 
 		/* Set the starting point of the paragraph. */
 		startline = openfile->current;