Add an ability to find out the current write location from a WPACKET

Reviewed-by: NRich Salz <rsalz@openssl.org>

ssl/packet.c

@@ -62,7 +62,8 @@ int WPACKET_reserve_bytes(WPACKET *pkt, size_t len, unsigned char **allocbytes)
         if (BUF_MEM_grow(pkt->buf, newlen) == 0)
             return 0;
     }
-    *allocbytes = GETBUF(pkt) + pkt->curr;
+    if (allocbytes != NULL)
+        *allocbytes = GETBUF(pkt) + pkt->curr;
 
     return 1;
 }
@@ -376,6 +377,11 @@ int WPACKET_get_length(WPACKET *pkt, size_t *len)
     return 1;
 }
 
+unsigned char *WPACKET_get_curr(WPACKET *pkt)
+{
+    return GETBUF(pkt) + pkt->curr;
+}
+
 void WPACKET_cleanup(WPACKET *pkt)
 {
     WPACKET_SUB *sub, *parent;

ssl/packet_locl.h

@@ -728,7 +728,7 @@ int WPACKET_start_sub_packet(WPACKET *pkt);
 /*
  * Allocate bytes in the WPACKET for the output. This reserves the bytes
  * and counts them as "written", but doesn't actually do the writing. A pointer
- * to the allocated bytes is stored in |*allocbytes|.
+ * to the allocated bytes is stored in |*allocbytes|. |allocbytes| may be NULL.
  * WARNING: the allocated bytes must be filled in immediately, without further
  * WPACKET_* calls. If not then the underlying buffer may be realloc'd and
  * change its location.
@@ -854,6 +854,12 @@ int WPACKET_get_total_written(WPACKET *pkt, size_t *written);
  */
 int WPACKET_get_length(WPACKET *pkt, size_t *len);
 
+/*
+ * Returns a pointer to the current write location, but does not allocate any
+ * bytes.
+ */
+unsigned char *WPACKET_get_curr(WPACKET *pkt);
+
 /* Release resources in a WPACKET if a failure has occurred. */
 void WPACKET_cleanup(WPACKET *pkt);