Simplify fill_gstring_header

* src/composite.c (fill_gstring_header): Omit first argument HEADER,
since in practice it is always nil.  Change caller to match.
Help the compiler by telling it LEN is nonnegative.
Problem found with --enable-gcc-warnings and gcc -O2 -Og.
1 job for master in 60 minutes and 1 second (queued for 2 seconds)
Status Job ID Name Coverage
  Test
failed #1256
test-all

01:00:01

 
Name Stage Failure
failed
test-all Test The script exceeded the maximum execution time set for the job
make -C test check-expensive
make[1]: Entering directory '/builds/emacs/emacs/test'
rm -f ./*.tmp
make[2]: Entering directory '/builds/emacs/emacs/test'
make[3]: Entering directory '/builds/emacs/emacs/test'
ELC lisp/net/tramp-tests.elc
GEN lisp/net/tramp-tests.log
Pulling docker image gitlab/gitlab-runner-helper:x86_64-f100a208 ...
ERROR: Job failed: execution took longer than 1h0m0s seconds