pass clang-ubsan

[mattdowle]

Fixes these observed locally, all in fread.c.
The nan one is a follow up to #4464.
Using clang-10 as per CRAN_Release.cmd. Does not show up under gcc-9, at least with the default options I used.

test.data.table() running: /home/mdowle/build/R-devel-strict-clang/library/data.table/tests/tests.Rraw 
Running test id 880      fread.c:992:15: runtime error: nan is outside the range of representable values of type 'int'
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior fread.c:992:15 in 
Running test id 899.1      fread.c:2273:43: runtime error: applying zero offset to null pointer
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior fread.c:2273:43 in 
Running test id 899.3      fread.c:2286:43: runtime error: applying zero offset to null pointer
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior fread.c:2286:43 in 
fread.c:2387:39: runtime error: applying zero offset to null pointer
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior fread.c:2387:39 in 

This will also fix the only 'additional issue' currently being reported on CRAN with 1.12.8 too :

test.data.table() running: /data/gannet/ripley/R/packages/tests-clang-SAN/data.table.Rcheck/data.table/tests/tests.Rraw.bz2 
  fread.c:2114:43: runtime error: applying zero offset to null pointer
... etc