Fix a false positive compiler warning/error on Alpine 3.24

On Alpine Linux 3.24, GCC 15 with fortify-headers produces a compiler
warning when building bin/nsupdate/nsupdate.c:

    In function 'fgets',
        inlined from 'get_next_command' at ../bin/nsupdate/nsupdate.c:2414:13:
    /usr/include/fortify/stdio.h:48:16: error: 'cmdlinebuf' may be used uninitialized [-Werror=maybe-uninitialized]
       48 |         return __orig_fgets(__s, __n, __f);
          |                ^~~~~~~~~~~~~~~~~~~~~~~~~~~
    /usr/include/fortify/stdio.h:42:1: note: in a call to '*fgets' declared with attribute 'access (read_write, 1, 2)' here
       42 | _FORTIFY_FN(fgets) char *fgets(char * _FORTIFY_POS0 __s, int __n, FILE *__f)
          | ^~~~~~~~~~~
    ../bin/nsupdate/nsupdate.c:2405:14: note: 'cmdlinebuf' declared here
     2405 |         char cmdlinebuf[MAXCMD];
          |              ^~~~~~~~~~

This is a false positive, because fgets() only writes into the buffer;
the fortify-headers wrapper annotates the buffer argument with
'access (read_write, ...)', which makes GCC treat passing an
uninitialized buffer as a read of uninitialized memory.

Initialize the 'cmdlinebuf' buffer anyway to avoid the build error.

Assisted-by: Claude:claude-fable-5
(cherry picked from commit 62670aa1b7)

bin/nsupdate/nsupdate.c

@@ -2432,7 +2432,7 @@ do_next_command(char *cmdline) {
 static uint16_t
 get_next_command(void) {
 	uint16_t result = STATUS_QUIT;
-	char cmdlinebuf[MAXCMD];
+	char cmdlinebuf[MAXCMD] = { 0 };
 	char *cmdline = NULL, *ptr = NULL;
 
 	if (interactive) {