fix catch symbol mangling

Only need to look up the immediate parent for the same-name special case.

fixes #1733

lib/scope.js

@@ -81,7 +81,7 @@ SymbolDef.prototype = {
             var def;
             if (options.screw_ie8
                 && sym instanceof AST_SymbolCatch
-                && (def = s.parent_scope.find_variable(sym))) {
+                && (def = s.parent_scope.variables.get(this.name))) {
                 this.mangled_name = def.mangled_name || def.name;
             } else
                 this.mangled_name = s.next_mangled(options, this);

test/compress/issue-1733.js

@@ -0,0 +1,41 @@
+function_iife_catch: {
+    mangle = {
+        screw_ie8: true,
+    }
+    input: {
+        function f(n) {
+            !function() {
+                try {
+                    throw 0;
+                } catch (n) {
+                    var a = 1;
+                    console.log(n, a);
+                }
+            }();
+        }
+        f();
+    }
+    expect_exact: "function f(o){!function(){try{throw 0}catch(c){var o=1;console.log(c,o)}}()}f();"
+    expect_stdout: "0 1"
+}
+
+function_iife_catch_ie8: {
+    mangle = {
+        screw_ie8: false,
+    }
+    input: {
+        function f(n) {
+            !function() {
+                try {
+                    throw 0;
+                } catch (n) {
+                    var a = 1;
+                    console.log(n, a);
+                }
+            }();
+        }
+        f();
+    }
+    expect_exact: "function f(o){!function(){try{throw 0}catch(o){var c=1;console.log(o,c)}}()}f();"
+    expect_stdout: "0 1"
+}