[exp] Fix awk sdev computation

bin/mc-2/exp

@@ -228,8 +228,8 @@ run_iterations() {
         printf '.'
     done
 
-    avg=$(awk '{ sum += $1 } END { if (NR > 0) print sum / NR; else print NaN }'     $exp_dir/*/time)
-    sd=$(awk '{ sum += $1; sum2 += $0^2 } END { print sqrt(sum2 / NR - (sum/NR)^2)}' $exp_dir/*/time)
+    avg=$(awk '{ sum += $1 } END { if (NR > 0) print sum / NR; else print NaN }' $exp_dir/*/time)
+    sd=$(awk '{ sum += $1; sum2 += $0^2 } END { print sqrt(abs(sum2 / NR - (sum/NR)^2)) } function abs(x){return ( x >= 0 ) ? x : -x }' $exp_dir/*/time)
     printf ' | Avg: %6.2f  ~%3.2f\n' $avg $sd
 }