ubsan warning at CRAN

[eddelbuettel]

See https://www.stats.ox.ac.uk/pub/bdr/memtests/clang-UBSAN/anytime/tests/allFormats.Rout:

> anytime(c("2016-09-01 10:11:12", "2016-09-01 10:11:12.345678"))
/data/gannet/ripley/R/test-clang/BH/include/boost/lexical_cast/detail/converter_lexical_streams.hpp:235:43: runtime error: downcast of address 0x7fffce329628 which does not point to an object of type 'buffer_t' (aka 'basic_unlockedbuf<std::basic_streambuf<char, char_traits<char> >, char>')
0x7fffce329628: note: object is of type 'std::__1::basic_stringbuf<char, std::__1::char_traits<char>, std::__1::allocator<char> >'
 9b 7f 00 00  50 15 42 38 9b 7f 00 00  80 01 46 46 9b 7f 00 00  00 00 00 00 00 00 00 00  00 00 00 00
              ^~~~~~~~~~~~~~~~~~~~~~~
              vptr for 'std::__1::basic_stringbuf<char, std::__1::char_traits<char>, std::__1::allocator<char> >'
SUMMARY: AddressSanitizer: undefined-behavior /data/gannet/ripley/R/test-clang/BH/include/boost/lexical_cast/detail/converter_lexical_streams.hpp:235:43 in 
[1] "2016-09-01 09:11:12.000000 BST" "2016-09-01 09:11:12.345678 BST"

Dismissed at first as a Boost issue; is actually related to the SEXP conversion.