Check argument count before accessing expression arguments

Fix #228

src/Compat.jl

@@ -269,34 +269,39 @@ function rewrite_pairs_to_tuples!(expr::Expr)
     return expr
 end
 
+function is_quote_symbol(ex::ANY, val::Symbol)
+    if isa(ex, QuoteNode)
+        return (ex::QuoteNode).value === val
+    elseif isa(ex, Expr)
+        ex = ex::Expr
+        return ex.head === :quote && length(ex.args) == 1 && ex.args[1] === val
+    end
+    return false
+end
+
 # rewrites accesses to IOContext dicts
 function rewrite_iocontext!(expr::Expr)
-    if expr.head == :call && expr.args[1] == :get
-        key = expr.args[3]
-        if     (((isa(key, QuoteNode) && key.value == :limit) ||
-                 (isa(key, Expr) && key.head == :quote && key.args[1] == :limit) ||
-                ((isa(key, QuoteNode) && key.value == :compact) ||
-                 (isa(key, Expr) && key.head == :quote && key.args[1] == :compact)))
-                && expr.args[4] == false)
+    args = expr.args
+    nargs = length(args)
+    if nargs == 4 && expr.head === :call && args[1] === :get && args[4] === false
+        key = args[3]
+        if is_quote_symbol(key, :limit) || is_quote_symbol(key, :compact)
             if VERSION >= v"0.5.0-dev+1936" && VERSION < v"0.5.0-dev+4305"
-                expr.args[1] = :(Base.limit_output)
-                deleteat!(expr.args, 3:4)
+                args[1] = :(Base.limit_output)
+                deleteat!(args, 3:4)
             elseif VERSION < v"0.5.0-dev+1936"
                 expr.head = :quote
-                expr.args[1] = false
-                deleteat!(expr.args, 3:4)
+                args[1] = false
+                deleteat!(args, 3:4)
             end
-        elseif (((isa(key, QuoteNode) && key.value == :multiline) ||
-                 (isa(key, Expr) && key.head == :quote && key.args[1] == :multiline))
-                && expr.args[4] == false)
+        elseif is_quote_symbol(key, :multiline)
             if VERSION < v"0.5.0-dev+4305"
                 expr.head = :quote
-                expr.args[1] = false
-                deleteat!(expr.args, 3:4)
+                args[1] = false
+                deleteat!(args, 3:4)
             end
         end
     end
-    expr
 end
 
 if VERSION < v"0.4.0-dev+707"

test/runtests.jl

@@ -1218,7 +1218,7 @@ io = IOBuffer()
 @test @compat(get(io, :limit, false)) == false
 @test @compat(get(io, :compact, false)) == false
 @test @compat(get(io, :multiline, false)) == false
-
+@test @compat(get(Nullable(1))) == 1
 
 let
     test_str = "test"