Using too long table aliases in stored routines might
have caused server crashes.

Code in sp_head::merge_table_list() which is responsible 
for collecting information about tables used in stored
routine was not aware of the fact that table alias might
have arbitrary length. I.e. it assumed that table alias
can't be longer than NAME_LEN bytes and allocated buffer
for a key identifying table accordingly.

This patch fixes the issue by ensuring that we use
dynamically allocated buffer for table key when table
alias is too long. By default stack based buffer is used
in which NAME_LEN bytes are reserved for table alias.