g.gui.vdigit: fix SnapLine function return value (#506)

gui/wxpython/vdigit/wxdigit.py

@@ -1165,7 +1165,7 @@ class IVDigit:
     def SnapLine(self):
         """Snap selected lines/boundaries
 
-        :return: on success
+        :return: 0 on success
         :return: -1 on error
         """
         if not self._checkMap():
@@ -1181,6 +1181,8 @@ class IVDigit:
             self._addChangeset()
             self.toolbar.EnableUndo()
 
+        return 0
+
     def ConnectLine(self):
         """Connect selected lines/boundaries