Only override the comma value on *_init if it is "invalid"

core/encoding/csv/writer.odin

@@ -17,7 +17,10 @@ Writer :: struct {
 
 // writer_init initializes a Writer that writes to w
 writer_init :: proc(writer: ^Writer, w: io.Writer) {
-	writer.comma = ','
+	switch writer.comma {
+	case '\x00', '\n', '\r', 0xfffd:
+		writer.comma = ','
+	}
 	writer.w = w
 }