helpers: MarshalToWriter: return jw.Error before writing

[omkhar]

Problem

MarshalToWriter writes the buffer to the io.Writer and returns a nil error even when encoding set jw.Error mid-stream. When a value sets w.Error (today: a +Inf/-Inf/NaN float, rejected since #421 / issue #134), the writer has already buffered a truncated/invalid document; DumpTo flushes those raw bytes to the destination and returns only its own (nil) error. The caller gets invalid/truncated JSON on the wire with no error.

The sibling MarshalToHTTPResponseWriter already gates on jw.Error before touching the writer; MarshalToWriter is the one wrapper that does not.

Fix

Mirror the sibling: return jw.Error before DumpTo, so no partial document is written and the caller is told.

• helpers.go: +3 lines.
• No-op for valid input (jw.Error is nil on success); strictly aligns MarshalToWriter's error contract with MarshalToHTTPResponseWriter.
• go test ./... passes.

Related: #134, #421 (the NaN/Inf guard that this path was never updated to honor).