app: check that GimpTool's display is present before actual commit.

A tool commit can be triggered in various cases, and the tool manager
relies on gimp_tool_has_display() to decide whether to run a tool
action. This function does much more than just checking GimpTool's
display. It also checks status_displays, and for a transform tool in
particular, it checks GimpDrawTool's display. This may be right for
other tools (I have no idea), so I can't just change this function.
Anyway we have to assume it is not a programming error if a transform
tool gets a COMMIT action while display is NULL (i.e. tool is halted).
When this happens, let's simply ignore.

This fixes the edge case raised by Ell, in comment 2 of bug 793150: when
an image has no layer, transform tools can't work and display is NULL.
But it still outputs status messages and therefore status_displays is
not empty. So the tool manager will still run a COMMIT action, which is
not an error. We only have to discard such COMMIT silently.

app/tools/gimptransformtool.c

@@ -317,7 +317,8 @@ gimp_transform_tool_control (GimpTool       *tool,
      break;
 
     case GIMP_TOOL_ACTION_COMMIT:
-      gimp_transform_tool_commit (tr_tool);
+      if (tool->display)
+        gimp_transform_tool_commit (tr_tool);
       break;
     }