objtool: Fix NULL printf() '%s' argument in builtin-check.c:save_argv()

It's probably not the best idea to pass a string pointer to printf()
right after confirming said pointer is NULL.  Fix the typo and use
argv[i] instead.

Fixes: c5995abe ("objtool: Improve error handling")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Tested-by: Stephen Rothwell <sfr@canb.auug.org.au>
Link: https://lore.kernel.org/r/a814ed8b08fb410be29498a20a5fbbb26e907ecf.1742952512.git.jpoimboe@kernel.org
Closes: https://lore.kernel.org/20250326103854.309e3c60@canb.auug.org.au