builtin: Fix echo -n early termination

The commit 7a784244 ("[BUILTIN]
Simplify echo command") broke echo -n by making it always terminate
after printing the first argument.

This patch fixes this by only terminating when we have reached
the end of the arguments.

Fixes: 7a784244 ("[BUILTIN] Simplify echo command")
Reported-by: Luigi Tarenga <luigi.tarenga@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

src/bltin/printf.c

@@ -459,7 +459,7 @@ echocmd(int argc, char **argv)
 		if (likely(*argv))
 			nonl += print_escape_str("%s", NULL, NULL, *argv++);
-		if (nonl > 0)
+		if (likely((nonl + !*argv) > 1))
 			break;
 		c = *argv ? ' ' : '\n';