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

Discovered using clang's MemorySanitizer when it ran python3's
test_fstring test_misformed_unicode_character_name.

An msan build will fail by simply executing: ./python -c 'u"\N"'
(cherry picked from commit 746b2d3)

Co-authored-by: Gregory P. Smith <[email protected]>

Author: miss-islington

Misc/NEWS.d/next/Core and Builtins/2018-11-13-00-40-35.bpo-35214.OQBjph.rst

@@ -0,0 +1,3 @@
+Fixed an out of bounds memory access when parsing a truncated unicode
+escape sequence at the end of a string such as ``'\N'``.  It would read
+one byte beyond the end of the memory allocation.

Objects/unicodeobject.c

@@ -6133,7 +6133,7 @@ _PyUnicode_DecodeUnicodeEscape(const char *s,
             }
 
             message = "malformed \\N character escape";
-            if (*s == '{') {
+            if (s < end && *s == '{') {
                 const char *start = ++s;
                 size_t namelen;
                 /* look for the closing brace */