Fix computing the size to clone for a body part

Take the optional header len into account also when cloning a body part struct.
This particular bug occured when using add_body_part() with extra hdr parameter in conjunction with an async() operation (or failure route)
Credits for reporting and helping with repoducing this go to @thuroc
Fixes #3067

(cherry picked from commit a042588)

parser/parse_body.c

@@ -509,7 +509,7 @@ int clone_sip_msg_body(struct sip_msg *src_msg, struct sip_msg *dst_msg,
 			extra_len = 0;
 		} else {
 			extra_len = (p->flags&SIP_BODY_PART_FLAG_NEW) ?
-				p->mime_s.len+p->body.len : 0 ;
+				p->mime_s.len+p->body.len+p->headers.len : 0 ;
 			if((np->next=func_malloc(my_malloc, sizeof(struct body_part)+extra_len))==NULL){
 				LM_ERR("failed to allocate new body_part clone (shared=%d)\n",
 					shared);