#3401 segfault fix

[arunsrinivasan]

Needs Calloc instead of R_alloc since we do a Realloc inside. Closes #3401.

[codecov]

Codecov Report

Merging #3404 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3404      +/-   ##
==========================================
+ Coverage    95.1%   95.12%   +0.01%     
==========================================
  Files          65       65              
  Lines       12126    12125       -1     
==========================================
+ Hits        11533    11534       +1     
+ Misses        593      591       -2

Impacted Files	Coverage Δ
src/uniqlist.c	96.68% <100%> (+1.12%)	⬆️
src/gsumm.c	90.88% <0%> (-0.03%)	⬇️

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