A potential bug of unchecked return value to null pointer dereference

[ash1852]

step 1 : In file flex/src/filter.c , function filter_tee_header line 245 : Function fdopen executes and stores the return value to to_c (to_c can be null) (lacking failure check, 2 out of 5 file openings checked failures)

step 2 : In file flex/src/filter.c , function filter_tee_header line 270 : to_c is used as the 2nd parameter in function fputs (to_c can be null)