Before this fix, the parser would sometime change where a token starts by
altering Lex_input_string::tok_start, which later confused the code in
sql_yacc.yy that needs to capture the source code of a SQL statement,
like to represent the body of a stored procedure.

This line of code in sql_lex.cc :

case MY_LEX_USER_VARIABLE_DELIMITER:
  lip->tok_start= lip->ptr; // Skip first `

would <skip the first back quote> ... and cause the bug reported.

In general, the responsibility of sql_lex.cc is to *find* where token are
in the SQL text, but is *not* to make up fake or incomplete tokens.
With a quoted label like `my_label`, the token starts on the first quote.
Extracting the token value should not change that (it did).

With this fix, the lexical analysis has been cleaned up to not change
lip->tok_start (in the case found for this bug).

The functions get_token() and get_quoted_token() now have an extra
parameters, used when some characters from the beginning of the token need
to be skipped when extracting a token value, like when extracting 'AB' from
'0xAB', for example, for a HEX_NUM token.

This exposed a bad assumption in Item_hex_string and Item_bin_string,
which has been fixed:

The assumption was that the string given, 'AB', was in fact preceded in
memory by '0x', which might be false (it can be preceded by "x'" and
followed by "'" -- or not be preceded by valid memory at all)

If a name is needed for Item_hex_string or Item_bin_string, the name is
taken from the original and true source code ('0xAB'), and assigned in
the select_item rule, instead of relying on assumptions related to how
memory is used.


mysql-test/r/sp.result:
  Lex_input_stream::tok_start must point at the real start of a token.
mysql-test/t/sp.test:
  Lex_input_stream::tok_start must point at the real start of a token.
sql/item.cc:
  Lex_input_stream::tok_start must point at the real start of a token.
sql/sql_lex.cc:
  Lex_input_stream::tok_start must point at the real start of a token.
sql/sql_yacc.yy:
  Lex_input_stream::tok_start must point at the real start of a token.