Fix small error in COPY FROM progress reporting.

The # of bytes processed was accumulated slightly incorrectly. After
loading more data to the input buffer, we added the number of bytes in
the buffer to the sum. But in case of multi-byte characters or escapes,
there can be a few unprocessed bytes left over from previous load in the
buffer. Those bytes got counted twice.

src/backend/commands/copyfromparse.c

@@ -386,7 +386,7 @@ CopyLoadRawBuf(CopyFromState cstate)
 	cstate->raw_buf[nbytes] = '\0';
 	cstate->raw_buf_index = 0;
 	cstate->raw_buf_len = nbytes;
-	cstate->bytes_processed += nbytes;
+	cstate->bytes_processed += inbytes;
 	pgstat_progress_update_param(PROGRESS_COPY_BYTES_PROCESSED, cstate->bytes_processed);
 	return (inbytes > 0);
 }