Port hbfont.c to GCC 9 with --enable-gcc-warnings

* src/hbfont.c (hbfont_shape): Use ptrdiff_t, not int,
for indexes that in theory might overflow int.
Add UNINIT to pacify GCC 9 and omit an unnecessary initialization.
1 job for harfbuzz in 2 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
failed #1971
test-all

00:00:02

 
Name Stage Failure
failed
test-all Test There has been a runner system failure, please try again
Running with gitlab-runner 11.6.0 (f100a208)
on gitlab-runner-docker cff9d373
Using Docker executor with image debian:stretch ...
Pulling docker image debian:stretch ...
Using docker image sha256:8d31923452f8b79ae91b01568d28c90e7d667a9eaff9734c6faeb017b0efa8d0 for debian:stretch ...
ERROR: Job failed (system failure): Error response from daemon: failed to start shim: exec: "docker-containerd-shim": executable file not found in $PATH: unknown (executor_docker.go:1018:0s)