MDEV-8281 aes_decrypt crashes in block_crypt()

fix aes_decrypt of yassl to support zero-length input

mysql-test/r/func_str.result

@@ -264,6 +264,9 @@ NULL
 select aes_decrypt(aes_encrypt("","a"),"a");
 aes_decrypt(aes_encrypt("","a"),"a")
 
+select aes_decrypt("", "a");
+aes_decrypt("", "a")
+NULL
 select repeat('monty',5),concat('*',space(5),'*');
 repeat('monty',5)	concat('*',space(5),'*')
 montymontymontymontymonty	*     *

mysql-test/t/func_str.test

@@ -105,6 +105,7 @@ select aes_decrypt(NULL,"a");
 select aes_decrypt("a",NULL);
 select aes_decrypt("a","a");
 select aes_decrypt(aes_encrypt("","a"),"a");
+select aes_decrypt("", "a");
 select repeat('monty',5),concat('*',space(5),'*');
 select reverse('abc'),reverse('abcd');
 select rpad('a',4,'1'),rpad('a',4,'12'),rpad('abcd',3,'12'), rpad(11, 10 , 22), rpad("ab", 10, 22);

mysys_ssl/my_crypt.cc

@@ -104,7 +104,7 @@ static int block_crypt(CipherMode cipher, Dir dir,
       }
       else
       {
-        int n= dest[source_length - 1];
+        int n= source_length ? dest[source_length - 1] : 0;
         if (tail || n == 0 || n > MY_AES_BLOCK_SIZE)
           return MY_AES_BAD_DATA;
         *dest_length-= n;