partedit: correct handling of vector fields

When the values input is shorter than the vector length then we need to
extend the values field so that we can we can replace it with its casted
value.

Author: David Aguilar

src/tools/partedit.py

@@ -1069,6 +1069,7 @@ def addAttributeSlot(self):
                 value = values[i]
             else:
                 value = values[-1]
+                values.append(value)
             if attrType == partio.INT or attrType == partio.INDEXEDSTR:
                 values[i] = int(value)
             elif attrType == partio.FLOAT or attrType == partio.VECTOR: