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:

    [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);
      |                                              ^~

This happens for usage() which passes NULL pointer as token. Normally
this is fine, as fprintf() will output "(null)" for the NULL pointer,
but it's still not ideal.

Fix the warning by changing the token to "" if it's 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;
 	}