CLEANUP: http: make it more obvious that msg->som is always null outside of chunks

Since the recent buffer reorg, msg->som is redundant with buf->p but still
appears at a number of places. This tiny patch allows to confirm that som
follows two states :
  - 0 from the moment the message starts to be parsed
  - relative offset to ->p for start of chunk when parsing chunks

During this second state, ->sol is never used, so we should probably merge
the two.
diff --git a/src/backend.c b/src/backend.c
index 3cc568c..9a5b196 100644
--- a/src/backend.c
+++ b/src/backend.c
@@ -260,8 +260,8 @@
 	const char      *params = b_ptr(req, (int)(msg->sov - req->o));
 	const char      *p    = params;
 
-	if (len > buffer_len(req) - (msg->sov - msg->som))
-		len = buffer_len(req) - (msg->sov - msg->som);
+	if (len > buffer_len(req) - msg->sov)
+		len = buffer_len(req) - msg->sov;
 
 	if (len == 0)
 		return NULL;
diff --git a/src/proto_http.c b/src/proto_http.c
index 5d522ab..05bfd47 100644
--- a/src/proto_http.c
+++ b/src/proto_http.c
@@ -1303,15 +1303,13 @@
 			 * first if we need to remove some CRLF. We can only
 			 * do this when o=0.
 			 */
-			char *beg = buf->p;
-
-			if (unlikely(ptr != beg)) {
+			if (unlikely(ptr != buf->p)) {
 				if (buf->o)
 					goto http_msg_ood;
 				/* Remove empty leading lines, as recommended by RFC2616. */
-				bi_fast_delete(buf, ptr - beg);
+				bi_fast_delete(buf, ptr - buf->p);
 			}
-			msg->sol = msg->som = ptr - buf->p;
+			msg->sol = msg->som = 0;
 			hdr_idx_init(idx);
 			state = HTTP_MSG_RPVER;
 			goto http_msg_rpver;
@@ -1370,15 +1368,13 @@
 			 * first if we need to remove some CRLF. We can only
 			 * do this when o=0.
 			 */
-			char *beg = buf->p;
-
-			if (likely(ptr != beg)) {
+			if (likely(ptr != buf->p)) {
 				if (buf->o)
 					goto http_msg_ood;
 				/* Remove empty leading lines, as recommended by RFC2616. */
-				bi_fast_delete(buf, ptr - beg);
+				bi_fast_delete(buf, ptr - buf->p);
 			}
-			msg->sol = msg->som = ptr - buf->p;
+			msg->sol = msg->som = 0;
 			/* we will need this when keep-alive will be supported
 			   hdr_idx_init(idx);
 			 */