btrfs-progs: fix usage warning in common/help.c

On systems with glibc 2.34 and 2.39, the following warning appears when
building the binary, causing concern that something has failed.

    [CC]     common/help.o
common/help.c: In function ‘usage’:
common/help.c:315:58: warning: ‘%s’ directive argument is null [-Wformat-overflow=]
  315 |                 fprintf(outf, "No short description for '%s'\n", token);
      |                                                          ^~
common/help.c:312:46: warning: ‘%s’ directive argument is null [-Wformat-overflow=]
  312 |                 fprintf(outf, "No usage for '%s'\n", token);
      |                                              ^~

The compiler warns that in some code paths, the token is `NULL`. Silence
the warning by checking if the token is `NULL`.

Reviewed-by: Qu Wenruo <[email protected]>
Signed-off-by: Anand Jain <[email protected]>

common/help.c

@@ -309,10 +309,10 @@ static int usage_command_internal(const char * const *usagestr,
 	ret = do_usage_one_command(usagestr, flags, cmd_flags, outf);
 	switch (ret) {
 	case -1:
-		fprintf(outf, "No usage for '%s'\n", token);
+		fprintf(outf, "No usage for '%s'\n", token ? : "");
 		break;
 	case -2:
-		fprintf(outf, "No short description for '%s'\n", token);
+		fprintf(outf, "No short description for '%s'\n", token ? : "");
 		break;
 	}