Issue #3263: fix critical error on bucket fill tool.

Fixes the error:
> Critical error: gimp_line_art_thaw: assertion 'line_art->priv->frozen'

This may happen in cases when we didn't actually freeze the line art at
pointer click, because we were in an invalid case (for instance,
clicking out of selection), hence we must not thaw the line art either
at button release.

(cherry picked from commit 6391b2bcff)

app/core/gimplineart.c

@@ -536,6 +536,12 @@ gimp_line_art_thaw (GimpLineArt *line_art)
     }
 }
 
+gboolean
+gimp_line_art_is_frozen (GimpLineArt *line_art)
+{
+  return line_art->priv->frozen;
+}
+
 GeglBuffer *
 gimp_line_art_get (GimpLineArt  *line_art,
                    gfloat      **distmap)

app/core/gimplineart.h

@@ -65,6 +65,7 @@ void                 gimp_line_art_set_input        (GimpLineArt  *line_art,
 GimpPickable       * gimp_line_art_get_input        (GimpLineArt  *line_art);
 void                 gimp_line_art_freeze           (GimpLineArt  *line_art);
 void                 gimp_line_art_thaw             (GimpLineArt  *line_art);
+gboolean             gimp_line_art_is_frozen        (GimpLineArt  *line_art);
 
 GeglBuffer         * gimp_line_art_get              (GimpLineArt  *line_art,
                                                      gfloat      **distmap);

app/tools/gimpbucketfilltool.c

@@ -420,7 +420,8 @@ gimp_bucket_fill_tool_halt (GimpBucketFillTool *tool)
 
   g_clear_object (&tool->priv->fill_mask);
 
-  gimp_line_art_thaw (tool->priv->line_art);
+  if (gimp_line_art_is_frozen (tool->priv->line_art))
+    gimp_line_art_thaw (tool->priv->line_art);
 
   GIMP_TOOL (tool)->display  = NULL;
   GIMP_TOOL (tool)->drawable = NULL;