null-dereference error on GCC 13

[Pietro Monteiro]

I'm building the tip of branch-1.4 (a886ea40a29a08954ff80772e267828a1d440cc9) 
with GCC 13 and it fails with a null-dereference error on output.c:491:

output.c: In function 'make_room_for':
output.c:491:40: error: potential null pointer dereference 
[-Werror=null-dereference]
  491 |       output_file = output_diversion->u.file;
         |                             ~~~~~~~~~~~~~~~~~~~^~~~~

If I tell GCC to stop caring about null-dereference errors I get a 
null-argument error on output.c:575:

$make CFLAGS="-Wno-error=null-dereference
snip...
output.c: In function 'output_text':
output.c:575:7: error: use of NULL where non-null expected [CWE-476] 
[-Werror=analyzer-null-argument]
  575 |       memcpy (output_cursor, text, (size_t) length);
          |       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
snip...
In file included from ../lib/string.h:41,
                 from m4.h:35,
                 from output.c:22:
/usr/include/string.h:43:14: note: argument 1 of 'memcpy' must be non-null
   43 | extern void *memcpy (void *__restrict __dest, const void *__restrict 
__src,
        |                     ^~~~~~

A simple NULL check on output_diversion at the start of make_room_for fixes 
both issues:

diff --git a/src/output.c b/src/output.c
index 6dc8280a..4bdf872b 100644
--- a/src/output.c
+++ b/src/output.c
@@ -416,6 +416,10 @@ make_room_for (int length)
      buffers start at 0 bytes, then 512, then keep doubling until it is
      decided to flush them to disk.  */

+  if (!output_diversion)
+      m4_failure (errno,
+                  _("ERROR: diversion not initialized"));
+
   output_diversion->used = output_diversion->size - output_unused;

   for (wanted_size = output_diversion->size;

Thanks,
pietro