libgimpbase: read up to 255 bytes into a 256 bytes buffer.

Otherwise if we add a NUL byte after the last byte, we might right past
the allocated memory. Thanks to Massimo for reporting this error raised
by Address Sanitizer and valgrind (cf. #7539).

libgimpbase/gimputils.c

@@ -1342,7 +1342,7 @@ gimp_stack_trace_print (const gchar   *prog_name,
        */
       close (out_fd[1]);
 
-      while ((read_n = read (out_fd[0], buffer, 256)) != 0)
+      while ((read_n = read (out_fd[0], buffer, 255)) != 0)
         {
           if (read_n < 0)
             {