Fix the internals of our hash functions to used unsigned values during hash computation as the overflow behavior of signed integers is undefined. In practice we require compiling everything with -fwrapv which forces overflow to be defined as twos compliment but this keeps the code cleaner for checkers or in the case where someone has compiled it without -fwrapv or their compiler's equivalent. Found by Clang trunk's Undefined Behavior Sanitizer (UBSan). Cleanup only - no functionality or hash values change.

--- a/Objects/bytesobject.c	Mon Dec 10 17:44:44 2012 -0800
+++ b/Objects/bytesobject.c	Mon Dec 10 18:15:46 2012 -0800
@@ -873,7 +873,7 @@
 {
     register Py_ssize_t len;
     register unsigned char *p;
-    register Py_hash_t x;
+    register Py_uhash_t x;  /* Unsigned for defined overflow behavior. */
 
 #ifdef Py_DEBUG
     assert(_Py_HashSecret_Initialized);