Avoid OP_VARARGPREP for active lines

when building the table 'activelines' for a vararg function, this
first instruction does not make the first line active.

ldebug.c

@@ -301,7 +301,14 @@ static void collectvalidlines (lua_State *L, Closure *f) {
     sethvalue2s(L, L->top, t);  /* push it on stack */
     api_incr_top(L);
     setbtvalue(&v);  /* boolean 'true' to be the value of all indices */
-    for (i = 0; i < p->sizelineinfo; i++) {  /* for all instructions */
+    if (!p->is_vararg)  /* regular function? */
+      i = 0;  /* consider all instructions */
+    else {  /* vararg function */
+      lua_assert(p->code[0] == OP_VARARGPREP);
+      currentline = nextline(p, currentline, 0);
+      i = 1;  /* skip first instruction (OP_VARARGPREP) */
+    }
+    for (; i < p->sizelineinfo; i++) {  /* for each instruction */
       currentline = nextline(p, currentline, i);  /* get its line */
       luaH_setint(L, t, currentline, &v);  /* table[line] = true */
     }

testes/db.lua

@@ -195,6 +195,49 @@ do   -- testing line info/trace with large gaps in source
   end
 end
 
+
+do   -- testing active lines
+  local function checkactivelines (f, lines)
+    local t = debug.getinfo(f, "SL")
+    for _, l in pairs(lines) do
+      l = l + t.linedefined
+      assert(t.activelines[l])
+      t.activelines[l] = undef
+    end
+    assert(next(t.activelines) == nil)   -- no extra lines
+  end
+
+  checkactivelines(function (...)   -- vararg function
+    -- 1st line is empty
+    -- 2nd line is empty
+    -- 3th line is empty
+    local a = 20
+    -- 5th line is empty
+    local b = 30
+    -- 7th line is empty
+  end, {4, 6, 8})
+
+  checkactivelines(function (a)
+    -- 1st line is empty
+    -- 2nd line is empty
+    local a = 20
+    local b = 30
+    -- 5th line is empty
+  end, {3, 4, 6})
+
+  checkactivelines(function (...) end, {0})
+
+  checkactivelines(function (a, b)
+  end, {1})
+
+  for _, n in pairs{0, 1, 2, 10, 50, 100, 1000, 10000} do
+    checkactivelines(
+      load(string.format("%s return 1", string.rep("\n", n))),
+      {n + 1})
+  end
+
+end
+
 print'+'
 
 -- invalid levels in [gs]etlocal