commands: return none, when offset exceeds file length

The computed length of the file may be negative, when the offset exceeds
the file length. When this is the case, return none

Change-Id: I2c017c620254fae188022851ef3b670730aab503

pySim/commands.py

@@ -155,6 +155,9 @@ class SimCardCommands(object):
 			return (None, None)
 		if length is None:
 			length = self.__len(r) - offset
+		if length < 0:
+			return (None, None)
+
 		total_data = ''
 		chunk_offset = 0
 		while chunk_offset < length: