core: Reset buffer state on new context alloc

We need to clear out old context data, otherwise we'll be repeating the
previous context's packet.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Change-Id: I5a1cc08afb7846da770e73edb7dafb65904ecb95
diff --git a/core.c b/core.c
index 560a4a7..baf60a8 100644
--- a/core.c
+++ b/core.c
@@ -167,6 +167,7 @@
 
 	ctx->src = src;
 	ctx->tag = tag;
+	ctx->buf_size = 0;
 
 	return ctx;
 }