This bug is also known as WL#1639.


mysql-test/r/func_group.result:
  Test case for stddev_pop, var_pop, stddev_samp, var_samp was added.
mysql-test/t/func_group.test:
  Test case for stddev_pop, var_pop, stddev_samp, var_samp was added.
sql/item_sum.cc:
  New functions stddev_samp and var_samp were added.
sql/item_sum.h:
  New functions stddev_samp and var_samp were added.
sql/lex.h:
  New functions stddev_pop, var_pop, stddev_samp and var_samp were added.
sql/sql_yacc.yy:
  New functions stddev_pop, var_pop, stddev_samp and var_samp were added.