head: fix subtraction underflow with --bytes=-N

Fix a subtraction underflow that occurred on `head --bytes=-N` when the
input had fewer than `N` bytes. Before this commit,

    printf "a" | head -c -2

would panic. After this commit, it terminates successfully with no
output as expected.

tests/by-util/test_head.rs

@@ -157,6 +157,23 @@ fn test_negative_byte_syntax() {
         .stdout_is("");
 }
 
+#[test]
+fn test_negative_bytes_greater_than_input_size_stdin() {
+    new_ucmd!()
+        .args(&["-c", "-2"])
+        .pipe_in("a")
+        .succeeds()
+        .no_output();
+}
+
+#[test]
+fn test_negative_bytes_greater_than_input_size_file() {
+    let ts = TestScenario::new(util_name!());
+    let at = &ts.fixtures;
+    at.write_bytes("f", b"a");
+    ts.ucmd().args(&["-c", "-2", "f"]).succeeds().no_output();
+}
+
 #[test]
 fn test_negative_zero_lines() {
     new_ucmd!()