Fixed memory exception when extra erroneous comma is present out-of-s…

[mattdowle]

Closes #2523
Thanks @st-pasha for great find and test.
Added test which I confirmed is detected by ASAN before the fix :

fread(f, header=FALSE)
=================================================================
==4281==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x60200007cad7 at pc 0x7f42b0dbb235 bp 0x7ffc940a8000 sp 0x7ffc940a7ff0
READ of size 1 at 0x60200007cad7 thread T0
#0 0x7f42b0dbb234 in freadMain /tmp/Rtmpf2iEIj/R.INSTALL7d51cfd676/data.table/src/fread.c:2002

[codecov-io]

Codecov Report

Merging #2570 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #2570   +/-   ##
======================================
  Coverage    91.4%   91.4%           
======================================
  Files          63      63           
  Lines       12097   12097           
======================================
  Hits        11057   11057           
  Misses       1040    1040

Impacted Files	Coverage Δ
src/fread.c	95.97% <100%> (ø)	⬆️

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