Added fuzz test for `Script::bytes_to_asm_fmt`

This adds fuzz target for `Script::bytes_to_asm_fmt` which could
panic due to overflow in the past. Fuzzing should decrease the risk of
other panics.

.github/workflows/fuzz.yml

@@ -11,7 +11,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        fuzz_target: [deser_net_msg, deserialize_address, deserialize_amount, deserialize_block, deserialize_psbt, deserialize_script, deserialize_transaction, outpoint_string, uint128_fuzz]
+        fuzz_target: [deser_net_msg, deserialize_address, deserialize_amount, deserialize_block, deserialize_psbt, deserialize_script, deserialize_transaction, outpoint_string, uint128_fuzz, script_bytes_to_asm_fmt]
     steps:
       - name: Install test dependencies
         run: sudo apt-get update -y && sudo apt-get install -y binutils-dev libunwind8-dev libcurl4-openssl-dev libelf-dev libdw-dev cmake gcc libiberty-dev

fuzz/Cargo.toml

@@ -55,3 +55,7 @@ path = "fuzz_targets/deser_net_msg.rs"
 [[bin]]
 name = "uint128_fuzz"
 path = "fuzz_targets/uint128_fuzz.rs"
+
+[[bin]]
+name = "script_bytes_to_asm_fmt"
+path = "fuzz_targets/script_bytes_to_asm_fmt.rs"

fuzz/fuzz_targets/script_bytes_to_asm_fmt.rs

@@ -0,0 +1,41 @@
+extern crate bitcoin;
+
+use std::fmt;
+
+// faster than String, we don't need to actually produce the value, just check absence of panics
+struct NullWriter;
+
+impl fmt::Write for NullWriter {
+    fn write_str(&mut self, _s: &str) -> fmt::Result {
+        Ok(())
+    }
+
+    fn write_char(&mut self, _c: char) -> fmt::Result {
+        Ok(())
+    }
+}
+
+fn do_test(data: &[u8]) {
+    let mut writer = NullWriter;
+    bitcoin::Script::bytes_to_asm_fmt(data, &mut writer);
+}
+
+#[cfg(feature = "afl")]
+#[macro_use] extern crate afl;
+#[cfg(feature = "afl")]
+fn main() {
+    fuzz!(|data| {
+        do_test(&data);
+    });
+}
+
+#[cfg(feature = "honggfuzz")]
+#[macro_use] extern crate honggfuzz;
+#[cfg(feature = "honggfuzz")]
+fn main() {
+    loop {
+        fuzz!(|data| {
+            do_test(data);
+        });
+    }
+}