Fix regression: line tool draws extra pixels on the canvas edge

How the bug could be reproduced (before this fix):
- Select the line tool.
- Click and hold down the left mouse button on the canvas to put the first line point.
- Drag the mouse outside the canvas and move it around (like trying to put the second point of the line outside the canvas).

src/app/tools/point_shape.cpp

@@ -1,5 +1,5 @@
 // Aseprite
-// Copyright (C)      2020  Igara Studio S.A.
+// Copyright (C) 2020-2021  Igara Studio S.A.
 // Copyright (C) 2001-2015  David Capello
 //
 // This program is distributed under the terms of
@@ -81,11 +81,11 @@ void PointShape::doInkHline(int x1, int y, int x2, ToolLoop* loop)
   }
   // Clipped in X axis
   else {
-    x1 = base::clamp(x1, 0, dstw-1);
-    x2 = base::clamp(x2, 0, dstw-1);
-    if (x2-x1+1 < 1)
+    if (x2 < 0 || x1 >= dstw || x2-x1+1 < 1)
       return;
 
+    x1 = base::clamp(x1, 0, dstw-1);
+    x2 = base::clamp(x2, 0, dstw-1);
     ink->inkHline(x1, y, x2, loop);
   }
 }