function_hooks: do the fake parameter assignment after parsing PARAM_LIMIT

function_hooks: do the fake parameter assignment after parsing PARAM_LIMIT

This was leading to false positive warnings about "we previously assumed
'ptr' could be NULL".  The situation is, say a function returns
"s64min-s64max[$0->foo]", then when we hit an INTERNAL record, which is
the first thing we hit for a return, then we have an assignment.  The
problem is that the next thing we hit is maybe a PARAM_LIMIT which says
that $0 is non-NULL.  So since we haven't processed the PARAM_LIMIT we
think maybe $0 is a potential NULL dereference.

The solution is to do the fake assignment last.

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