m4 hangup in 6.0_BETA

[Edgar Fuß <ef%math.uni-bonn.de@localhost>]

In 6.0_BETA the following simple input causes M4 to lock up:

ifdef(`FOO',
# FOO
)

It doesn't matter whether FOO is defined or not.
If one replaces the comment by something else, it does work.

Should I file a PR?

Please CC me on replies since I'm not subscribed to tech-toolchain.
Thanks.