cranelift-object: reject symbol names with NUL byte without panic'ing.

Avoid a `panic!()`, and return a proper error, on a NUL byte. We hit a
null-byte check inside the `object` crate otherwise; this blocks fuzzing
when testing via a write-object-file-and-dlopen flow.

cranelift/object/tests/basic.rs

@@ -197,3 +197,46 @@ fn libcall_function() {
 
     module.finish();
 }
+
+#[test]
+#[should_panic(
+    expected = "Result::unwrap()` on an `Err` value: Backend(Symbol \"function\\u{0}with\\u{0}nul\\u{0}bytes\" has a null byte, which is disallowed"
+)]
+fn reject_nul_byte_symbol_for_func() {
+    let flag_builder = settings::builder();
+    let isa_builder = cranelift_codegen::isa::lookup_by_name("x86_64-unknown-linux-gnu").unwrap();
+    let isa = isa_builder.finish(settings::Flags::new(flag_builder));
+    let mut module =
+        ObjectModule::new(ObjectBuilder::new(isa, "foo", default_libcall_names()).unwrap());
+
+    let sig = Signature {
+        params: vec![],
+        returns: vec![],
+        call_conv: CallConv::SystemV,
+    };
+
+    let _ = module
+        .declare_function("function\u{0}with\u{0}nul\u{0}bytes", Linkage::Local, &sig)
+        .unwrap();
+}
+
+#[test]
+#[should_panic(
+    expected = "Result::unwrap()` on an `Err` value: Backend(Symbol \"data\\u{0}with\\u{0}nul\\u{0}bytes\" has a null byte, which is disallowed"
+)]
+fn reject_nul_byte_symbol_for_data() {
+    let flag_builder = settings::builder();
+    let isa_builder = cranelift_codegen::isa::lookup_by_name("x86_64-unknown-linux-gnu").unwrap();
+    let isa = isa_builder.finish(settings::Flags::new(flag_builder));
+    let mut module =
+        ObjectModule::new(ObjectBuilder::new(isa, "foo", default_libcall_names()).unwrap());
+
+    let _ = module
+        .declare_data(
+            "data\u{0}with\u{0}nul\u{0}bytes",
+            Linkage::Local,
+            /* writable = */ true,
+            /* tls = */ false,
+        )
+        .unwrap();
+}