flow: expressions: don't try updating to uninitialized position

flow: expressions: don't try updating to uninitialized position

smatch_expression.c is designed to take a number and create a struct
expression of EXPR_VALUE for that number.  The problem is what position
should we use?

I guess we should try to use the position closest to what we're working on
right now.  I use the big_expression_stack and the big_statement_stack for
this.

The problem is that sometimes we still don't know.  If set_position() gets
a position with pos.stream == 0 and pos.line == 0 then it does bogus stuff.
Just ignore those values.

This is an ugly hack piece of crap code, obviously.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>