bugfix for cursor arguments in named notation

Using a cursor argument name equal to another plpgsql variable results 
in the error:
cursor .. has no argument named ""

The attached patch fixes that.

Instead of solving the issue like is done in the patch, another way 
would be to expose internal_yylex() so that could be used instead of 
yylex() by read_cursor_args when reading the argument name, and would 
always return the argument name in yylval.str.

-- 
Yeb Havinga
http://www.mgrid.net/
Mastering Medical Data