supersede: Fix crash when malloc() fails

[Bruno Haible]

Compiling a testdir with CC="gcc -fanalyzer" produced this warning:

gllib/supersede.c:63:3: warning: use of possibly-NULL 'temp_filename' where 
non-null expected [CWE-690] [-Wanalyzer-possible-null-argument]


2021-04-30  Bruno Haible  <br...@clisp.org>

        supersede: Fix crash when malloc() fails.
        Found by GCC 11 '-fanalyzer'.
        * lib/supersede.c (create_temp_file): Don't crash when malloc() fails.

diff --git a/lib/supersede.c b/lib/supersede.c
index 7371e20..61b4f04 100644
--- a/lib/supersede.c
+++ b/lib/supersede.c
@@ -60,6 +60,8 @@ create_temp_file (char *canon_filename, int flags, mode_t 
mode,
   /* The temporary file needs to be in the same directory, otherwise the
      final rename may fail.  */
   char *temp_filename = (char *) malloc (canon_filename_length + 7 + 1);
+  if (temp_filename == NULL)
+    return -1;
   memcpy (temp_filename, canon_filename, canon_filename_length);
   memcpy (temp_filename + canon_filename_length, ".XXXXXX", 7 + 1);