clear gcc warnings

[mattdowle]

Follow up to #4289
Clear gcc warnings all like this one:

gcc -I"/usr/share/R/include" -DNDEBUG -fpic -fopenmp -std=c99 -O3 -pipe -Wall -pedantic -fno-common -c fifelse.c -o fifelse.o
fifelse.c: In function ‘fifelseR’:
fifelse.c:188:45: warning: ‘pa’ may be used uninitialized in this function [-Wmaybe-uninitialized]
  188 |         if (!na_a) SET_VECTOR_ELT(ans, i, pa[i & amask]);

[codecov]

Codecov Report

Merging #5001 (6bdbb73) into master (5cc3455) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #5001   +/-   ##
=======================================
  Coverage   99.47%   99.47%           
=======================================
  Files          75       75           
  Lines       14775    14775           
=======================================
  Hits        14697    14697           
  Misses         78       78           

Impacted Files	Coverage Δ
src/fifelse.c	100.00% <100.00%> (ø)

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