Initialize data->fn_rows in rbindlist (fixes #2019)

[mllg]

data.fn_rows is initialized here:
https://github.com/Rdatatable/data.table/blob/master/src/rbindlist.c#L505

However, this loop has a conditional break, resulting in data.fn_rows[i] being undefined if not set in
https://github.com/Rdatatable/data.table/blob/master/src/rbindlist.c#L524
which again is only executed conditionally (continue in L517+L519).

This may lead to a segfault while writing values to the id column in https://github.com/Rdatatable/data.table/blob/master/src/rbindlist.c#L770
and https://github.com/Rdatatable/data.table/blob/master/src/rbindlist.c#L778.

[codecov-io]

Codecov Report

Merging #2077 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2077      +/-   ##
==========================================
+ Coverage   90.58%   90.61%   +0.03%     
==========================================
  Files          58       58              
  Lines       11055    11055              
==========================================
+ Hits        10014    10018       +4     
+ Misses       1041     1037       -4

Impacted Files	Coverage Δ
src/rbindlist.c	88.93% <100%> (ø)	⬆️
src/forder.c	94.47% <0%> (+0.52%)	⬆️

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