Add fuzzer for the encoder

fuzz/fuzz_targets/qoi_encode.rs

@@ -0,0 +1,29 @@
+#![no_main]
+use libfuzzer_sys::fuzz_target;
+
+fuzz_target!(|input: (bool, u8, &[u8])| {
+    let (is_4, w_frac, data) = input;
+    let channels = if is_4 { 4 } else { 3 };
+    let size = data.len();
+    let n_pixels = size / channels as usize;
+    let (w, h) = if n_pixels == 0 {
+        (0, 0)
+    } else {
+        let w = ((n_pixels * (1 + w_frac as usize)) / 256).max(1);
+        let h = n_pixels / w;
+        (w, h)
+    };
+    let out = qoi_fast::qoi_encode_to_vec(
+        &data[..(w * h * channels as usize)],
+        w as u32,
+        h as u32,
+        channels,
+        0,
+    );
+    if w * h != 0 {
+        let out = out.unwrap();
+        assert!(out.len() <= qoi_fast::encode_size_required(w as u32, h as u32, channels));
+    } else {
+        assert!(out.is_err());
+    }
+});