[2.7] bpo-35214: Fix OOB memory access in unicode escape parser (GH-10506) (GH-10538)

Discovered using clang's MemorySanitizer.

A msan build will fail by simply executing: ./python -c 'u"\N"'
(cherry picked from commit 746b2d35)

Co-authored-by: Gregory P. Smith <greg@krypto.org> [Google LLC]

Misc/NEWS.d/next/Core and Builtins/2018-11-13-17-20-18.bpo-35214.AH2F87.rst

+Fixed an out of bounds memory access when parsing a truncated unicode escape
+sequence at the end of a string such as ``u'\N'``.  It would read one byte
+beyond the end of the memory allocation.

Objects/unicodeobject.c

@@ -2950,7 +2950,7 @@ PyObject *PyUnicode_DecodeUnicodeEscape(const char *s,
                 if (ucnhash_CAPI == NULL)
                     goto ucnhashError;
             }
-            if (*s == '{') {
+            if (s < end && *s == '{') {
                 const char *start = s+1;
                 /* look for the closing brace */
                 while (*s != '}' && s < end)