Optimize merging line coverage

Prior this change `Integer#to_i` and `NilClass#to_i` showed up in
stack profiler which resulted to slow down `merge_line_coverage` a lot.

This change brought down the run time from 1m28s to 1m18s on a large
resultset (GitLab).

lib/simplecov/combine/lines_combiner.rb

@@ -30,12 +30,12 @@ def combine(coverage_a, coverage_b)
       #
       # @return [Integer || nil]
       def merge_line_coverage(first_val, second_val)
-        sum = first_val.to_i + second_val.to_i
-
-        if sum.zero? && (first_val.nil? || second_val.nil?)
-          nil
-        else
-          sum
+        if first_val && second_val
+          first_val + second_val
+        elsif first_val
+          first_val.zero? ? nil : first_val
+        elsif second_val
+          second_val.zero? ? nil : second_val
         end
       end
     end