Fix jsgen bug with uninitialized seq (#12500) [backport]

compiler/jsgen.nim

@@ -1169,7 +1169,7 @@ proc genArrayAddr(p: PProc, n: PNode, r: var TCompRes) =
     first = firstOrd(p.config, typ.sons[0])
   if optBoundsCheck in p.options:
     useMagic(p, "chckIndx")
-    r.res = "chckIndx($1, $2, $3.length+$2-1)-$2" % [b.res, rope(first), tmp]
+    r.res = "chckIndx($1, $2, ($3 != null ? $3.length : 0)+$2-1)-$2" % [b.res, rope(first), tmp]
   elif first != 0:
     r.res = "($1)-$2" % [b.res, rope(first)]
   else:

tests/js/t12223.nim

@@ -0,0 +1,20 @@
+discard """
+  action: "run"
+  output: '''
+caught
+index out of bounds, the container is empty
+'''
+"""
+
+proc fun() =
+  var z: seq[string]
+  discard z[4]
+
+proc main()=
+  try:
+    fun()
+  except Exception as e:
+    echo "caught"
+    echo getCurrentExceptionMsg()
+
+main()