Avoid double-move

This was flagged by coverity but doesn't seem to be an actual issue in
g++/clang.  It technically leaves the moved rvalue in a "valid but
undefined state", so it is best to avoid.  The single copy into an
lvalue is (I think) cheap

thirdparty/thread-pool/bs_thread_pool.hpp

@@ -405,8 +405,9 @@ public:
         blocks blks(first_index, index_after_last, num_blocks ? num_blocks : thread_count);
         if (blks.get_total_size() > 0)
         {
+            F internal_loop = loop;
             for (size_t i = 0; i < blks.get_num_blocks(); ++i)
-                push_task(std::forward<F>(loop), blks.start(i), blks.end(i));
+                push_task(std::forward<F>(internal_loop), blks.start(i), blks.end(i));
         }
     }