[WASM] Fix interop adapter for long strings

compiler/testData/codegen/boxWasmJsInterop/longStrings.kt

@@ -0,0 +1,19 @@
+// FILE: externals.js
+
+function id(str) {
+    return str
+}
+
+// FILE: externals.kt
+
+external fun id(str: String): String
+
+fun box(): String {
+    var x = "1234567890"
+    for (i in 1 until 20) {
+        x += x
+        val stringFromJs = id(x)
+        if (!stringFromJs.equals(x)) return "FAIL"
+    }
+    return "OK"
+}