Fix R function call in tryXts C function

Evaluating the expression created by LCONS() would cause a crash in
RcppXts because the second argument to LCONS should be a pairlist, not
a SEXP. This bug was introducted in the commit that removed the
SET_TYPEOF() calls.

inst/tinytest/test-reclass.R

@@ -50,3 +50,9 @@ xts_attr <- list("hello" = "world")
 xtsAttributes(y) <- xts_attr
 z <- reclass(x, y)
 expect_equal(xts_attr, xtsAttributes(z), info = info_msg)
+
+# ensure calls to tryXts() C function work
+x <- xts(1:3, .Date(1:3))
+z <- .Call(xts:::C_tryXts, as.zoo(x))
+y <- try.xts(as.zoo(x))
+expect_identical(y, z, info = "calls to C_tryXts() call try.xts()")

src/tryXts.c

@@ -33,7 +33,7 @@ SEXP tryXts (SEXP x)
     PROTECT(str_xts = mkString("xts")); P++;
     PROTECT(env = R_FindNamespace(str_xts)); P++;
 
-    PROTECT(expr = LCONS(install("try.xts"), x)); P++;
+    PROTECT(expr = lang2(install("try.xts"), x)); P++;
     PROTECT(result = eval(expr, env)); P++;
 
     if( !Rf_asInteger(isXts(result)) ) {