printf: error on missing hexadecial escape value

Change `printf` to correctly terminate with an error message when an
escape sequence starts with `\x` but doesn't include a literal
hexadecimal value after. For example, before this commit,

    printf '\x'

would output `\x`, but after this commit, it terminates with an error
message,

    printf: missing hexadecimal number in escape

Fixes #7097

tests/by-util/test_printf.rs

@@ -46,6 +46,15 @@ fn escaped_hex() {
     new_ucmd!().args(&["\\x41"]).succeeds().stdout_only("A");
 }
 
+#[test]
+fn test_missing_escaped_hex_value() {
+    new_ucmd!()
+        .arg(r"\x")
+        .fails()
+        .code_is(1)
+        .stderr_only("printf: missing hexadecimal number in escape\n");
+}
+
 #[test]
 fn escaped_octal() {
     new_ucmd!().args(&["\\101"]).succeeds().stdout_only("A");