nak: Only convert the written portion of the buffer in NirInstrPrinter

Fixes: 02774be708 ("nak/sm50: add a memstream abstraction")
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/29591>

src/nouveau/compiler/nak/nir_instr_printer.rs

@@ -32,8 +32,11 @@ impl NirInstrPrinter {
         unsafe {
             let stream = self.stream.as_mut().get_unchecked_mut();
             nak_nir_asprint_instr(stream, instr);
-            let c_str = std::ffi::CStr::from_ptr(stream.buffer);
-            let string = c_str.to_string_lossy().into_owned();
+            let bytes = std::slice::from_raw_parts(
+                stream.buffer as *const u8,
+                stream.written,
+            );
+            let string = String::from_utf8_lossy(bytes).into_owned();
             nak_clear_memstream(stream);
             string
         }