Poly1305: tolerate the intentional unsigned wraparound in poly1305.cpp

test/sanitizer_suppressions/ubsan

@@ -15,6 +15,7 @@ unsigned-integer-overflow:coded_stream.h
 unsigned-integer-overflow:core_write.cpp
 unsigned-integer-overflow:crypto/chacha20.cpp
 unsigned-integer-overflow:crypto/ctaes/ctaes.c
+unsigned-integer-overflow:crypto/poly1305.cpp
 unsigned-integer-overflow:crypto/ripemd160.cpp
 unsigned-integer-overflow:crypto/sha1.cpp
 unsigned-integer-overflow:crypto/sha256.cpp