MDEV-35717: UBSAN: runtime errors: applying [zero|non-zero] offset to null pointer

Problem:
  UBSAN reports runtime errors in string comparision functions where
  pointer arithmetic is done without checking NULL.

Fix:
  Check pointers are not NULL before doing pointer arithmetic.

Author: raghunandanbhat

mysql-test/main/t.result

@@ -0,0 +1,7 @@
+#
+# MDEV-35717: UBSAN: runtime errors: applying zero offset to null pointer in `my_strnncoll_utf8mb3_general1400_as_ci`, and applying non-zero offset 4 to null pointer in `my_strcoll_ascii_4bytes_found`
+#
+CREATE TABLE t (c INT,c2 CHAR,c3 DATE,CHECK (c>0));
+ALTER TABLE t ADD INDEX (c2) USING HASH;
+DROP TABLE t;
+# End of 11.8 tests

mysql-test/main/t.test

@@ -0,0 +1,10 @@
+--echo #
+--echo # MDEV-35717: UBSAN: runtime errors: applying zero offset to null pointer in `my_strnncoll_utf8mb3_general1400_as_ci`, and applying non-zero offset 4 to null pointer in `my_strcoll_ascii_4bytes_found`
+--echo #
+
+CREATE TABLE t (c INT,c2 CHAR,c3 DATE,CHECK (c>0));
+ALTER TABLE t ADD INDEX (c2) USING HASH;
+
+DROP TABLE t;
+
+--echo # End of 11.8 tests

strings/strcoll.inl

@@ -226,8 +226,8 @@ MY_FUNCTION_NAME(strnncoll)(CHARSET_INFO *cs __attribute__((unused)),
                             const uchar *b, size_t b_length,
                             my_bool b_is_prefix)
 {
-  const uchar *a_end= a + a_length;
-  const uchar *b_end= b + b_length;
+  const uchar *a_end= a ? a + a_length : NULL;
+  const uchar *b_end= b ? b + b_length : NULL;
   for ( ; ; )
   {
     int a_weight, b_weight, res;