py/modstruct: Allow to have "0s" in struct format.

tests/basics/struct2.py

@@ -0,0 +1,28 @@
+# test ustruct with a count specified before the type
+
+try:
+    import ustruct as struct
+except:
+    import struct
+
+print(struct.calcsize('0s'))
+print(struct.unpack('0s', b''))
+print(struct.pack('0s', b'123'))
+
+print(struct.calcsize('2s'))
+print(struct.unpack('2s', b'12'))
+print(struct.pack('2s', b'123'))
+
+print(struct.calcsize('2H'))
+print(struct.unpack('<2H', b'1234'))
+print(struct.pack('<2H', 258, 515))
+
+print(struct.calcsize('0s1s0H2H'))
+print(struct.unpack('<0s1s0H2H', b'01234'))
+print(struct.pack('<0s1s0H2H', b'abc', b'abc', 258, 515))
+
+# check that zero of an unknown type raises an exception
+try:
+    struct.calcsize('0z')
+except:
+    print('Exception')