py/binary: Change internal bytearray typecode from 0 to 1.

The value of 0 can't be used because otherwise mp_binary_get_size will let
a null byte through as the type code (intepreted as byterray).  This can
lead to invalid type-specifier strings being let through without an error
in the struct module, and even buffer overruns.

tests/basics/struct2.py

@@ -40,3 +40,30 @@ try:
     struct.calcsize('0z')
 except:
     print('Exception')
+
+# check that a count without a type specifier raises an exception
+
+try:
+    struct.calcsize('1')
+except:
+    print('Exception')
+
+try:
+    struct.pack('1')
+except:
+    print('Exception')
+
+try:
+    struct.pack_into('1', bytearray(4), 0, 'xx')
+except:
+    print('Exception')
+
+try:
+    struct.unpack('1', 'xx')
+except:
+    print('Exception')
+
+try:
+    struct.unpack_from('1', 'xx')
+except:
+    print('Exception')