fix corner case in merge_vars (#5421)

fixes #5420

lib/compress.js

@@ -6246,9 +6246,7 @@ Compressor.prototype.compress = function(node) {
             if (node instanceof AST_Try) {
                 var save_try = in_try;
                 in_try = node;
-                var save = segment;
                 walk_body(node, tw);
-                segment = save;
                 if (node.bcatch) {
                     if (node.bcatch.argname) node.bcatch.argname.mark_symbol(function(node) {
                         if (node instanceof AST_SymbolCatch) {
@@ -6266,7 +6264,6 @@ Compressor.prototype.compress = function(node) {
                     }
                 }
                 in_try = save_try;
-                segment = save;
                 if (node.bfinally) node.bfinally.walk(tw);
                 return true;
             }

test/compress/merge_vars.js

@@ -3702,3 +3702,33 @@ issue_5182: {
     ]
     node_version: ">=4"
 }
+
+issue_5420: {
+    options = {
+        merge_vars: true,
+        toplevel: true,
+    }
+    input: {
+        do {
+            var a = "FAIL 1";
+            a && a.p;
+            a = "FAIL 2";
+            try {
+                continue;
+            } catch (e) {}
+            var b = "FAIL 3";
+        } while (console.log(b || "PASS"));
+    }
+    expect: {
+        do {
+            var a = "FAIL 1";
+            a && a.p;
+            a = "FAIL 2";
+            try {
+                continue;
+            } catch (e) {}
+            var b = "FAIL 3";
+        } while (console.log(b || "PASS"));
+    }
+    expect_stdout: "PASS"
+}