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.

py/binary.h

@@ -29,8 +29,9 @@
 #include "py/obj.h"
 
 // Use special typecode to differentiate repr() of bytearray vs array.array('B')
-// (underlyingly they're same).
+// (underlyingly they're same).  Can't use 0 here because that's used to detect
-#define BYTEARRAY_TYPECODE 0
+// type-specification errors due to end-of-string.
+#define BYTEARRAY_TYPECODE 1
 
 size_t mp_binary_get_size(char struct_type, char val_type, mp_uint_t *palign);
 mp_obj_t mp_binary_get_val_array(char typecode, void *p, mp_uint_t index);

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')