fix for if (...) return; else return ...;

(it was assumed that the first `return` always contains a value)

close #22

test/compress/issue-22.js

@@ -0,0 +1,17 @@
+return_with_no_value_in_if_body: {
+    options = { conditionals: true };
+    input: {
+        function foo(bar) {
+            if (bar) {
+                return;
+            } else {
+                return 1;
+            }
+        }
+    }
+    expect: {
+        function foo (bar) {
+            return bar ? void 0 : 1;
+        }
+    }
+}