Fixes this compiler warning:

  read.c: In function 'on_process':
  read.c:402:38: warning: comparison between pointer and zero character constant [-Wpointer-compare]
       notnul < buf+thisrecsz && notnul == '\0';
					^~
  read.c:402:31: note: did you mean to dereference the pointer?
       notnul < buf+thisrecsz && notnul == '\0';

History of this fix: See #579604.  I am tidying up my own
program (innduct), which I find it still has a copy of this file.
Comparing the files I find this one difference.  I seem to have fixed
this bug in 2022 without noticing that I ought to be sending the fix
upstream.  So, apologies for the delay reporting this.

Patch attached.

Thanks,
Ian.