BUG/MINOR: http: error snapshots are wrong if buffer wraps

Commit 81f2fb added support for wrapping buffer captures, but unfortunately
the code used to perform two memcpy() over the same destination, causing a
loss of the start of the buffer rendering some error snapshots unusable.

This bug is present in 1.4 too and must be backported.
diff --git a/src/proto_http.c b/src/proto_http.c
index 2c1ff56..922ff61 100644
--- a/src/proto_http.c
+++ b/src/proto_http.c
@@ -7227,7 +7227,7 @@
 		es->len = bi_end(buf) - (buf->p + msg->som) + buf->size;
 		memcpy(es->buf, buf->p + msg->som, MIN(len1, sizeof(es->buf)));
 		if (es->len > len1 && len1 < sizeof(es->buf))
-			memcpy(es->buf, buf->data, MIN(es->len, sizeof(es->buf)) - len1);
+			memcpy(es->buf + len1, buf->data, MIN(es->len, sizeof(es->buf)) - len1);
 	}
 	else {
 		es->len = bi_end(buf) - (buf->p + msg->som);