src: fix null deref in AllocatedBuffer::clear

An empty buffer can have a null environment.  Previously, we were
getting away with with this, but -fsanitize=null in clang caught it.

PR-URL: #32892
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: Jan Krems <jan.krems@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Juan José Arboleda <soyjuanarbol@gmail.com>

src/env-inl.h

@@ -1003,7 +1003,10 @@ inline AllocatedBuffer::~AllocatedBuffer() {
 
 inline void AllocatedBuffer::clear() {
   uv_buf_t buf = release();
-  env_->Free(buf.base, buf.len);
+  if (buf.base != nullptr) {
+    CHECK_NOT_NULL(env_);
+    env_->Free(buf.base, buf.len);
+  }
 }
 
 // It's a bit awkward to define this Buffer::New() overload here, but it