poly1305: fix bug in amd64 assembly

Add the conditional subtraction of 3 from 'h2' (register R10).

Change-Id: I75615b0375f050a5cd97b968075c2992ccd1dee7
Reviewed-on: https://go-review.googlesource.com/30101
Reviewed-by: Adam Langley <agl@golang.org>

poly1305/poly1305_amd64.s

@@ -121,6 +121,7 @@ done:
 	MOVQ    R9, BX
 	SUBQ    $0xFFFFFFFFFFFFFFFB, AX
 	SBBQ    $0xFFFFFFFFFFFFFFFF, BX
+	SBBQ    $3, R10
 	CMOVQCS R8, AX
 	CMOVQCS R9, BX
 	ADDQ    16(SP), AX