Remove warning about tail "potentially uninitialized"

MSVC was complaining that the `tail` variable was potentially
uninitialized in the while branch. Since the while loop is actually
coupled to the if (head != NULL) that initializes the tail variable,
we move them together, which makes the warning disappear.

src/datastructures.c

@@ -52,7 +52,6 @@ HSlist* h_slist_copy(HSlist *slist) {
     h_slist_push(ret, head->elem);
     tail = ret->head;
     head = head->next;
-  }
     while (head != NULL) {
       // append head item to tail in a new node
       HSlistNode *node = h_arena_malloc(slist->arena, sizeof(HSlistNode));
@@ -61,6 +60,7 @@ HSlist* h_slist_copy(HSlist *slist) {
       tail = tail->next = node;
       head = head->next;
     }
+  }
   return ret;
 }
 

tools/windows/hammer_lib_src_list

@@ -4,6 +4,7 @@ benchmark.c
 bitreader.c 
 bitwriter.c 
 cfgrammar.c
+datastructures.c
 desugar.c 
 glue.c 
 hammer.c