Don't use bio_writer_t.skip() to write length field when appending more data

If the writer reallocates its buffer, the length pointer might not be valid
anymore, or even worse, point to an arbitrary allocation.

src/libstrongswan/bio/bio_writer.h

@@ -126,8 +126,11 @@ struct bio_writer_t {
 	void (*wrap32)(bio_writer_t *this);
 
 	/**
-	 * Skips len bytes in the buffer before the next data is written, returns
-	 * a chunk covering the skipped bytes.
+	 * Skips len bytes in the buffer, return chunk of skipped data.
+	 *
+	 * The returned chunk is not valid after calling any other writer function
+	 * (except get_buf()), because a buffer reallocation might move the
+	 * internal buffer to a different memory location!
 	 *
 	 * @param len		number of bytes to skip
 	 * @return			chunk pointing to skipped bytes in the internal buffer