fixed compilation warning.

src/CountMinSketch.hpp

@@ -190,7 +190,7 @@ template<typename T> inline int CountMinSketch<T>::isRCovBelowThres(const readNu
             h=hash64to32(*it,j);
             min=cms_lambda_array[j] [h];
         }
-        (min<threshold)? a+=1: NULL;
+        (min<threshold)? a+=1:a;
         ++b;
     }
     return (2*a>b);