Fix illegal read in do_is_ordered

do_is_ordered tries to check whether the first element of the series
is NA.  It needs to first check whether the series has at least one
element to avoid reading an uninitialised value.

Fixes joshuaulrich#236.

src/isOrdered.c

@@ -31,6 +31,12 @@ SEXP do_is_ordered (SEXP x, SEXP increasing, SEXP strictly)
   double *real_x;
   int *int_x;
 
+  /*
+  If length is 0 then it is ordered
+  */
+  if (nx < 0)
+    return ScalarLogical(1);
+
   if(TYPEOF(x) == REALSXP) {
   /*
   Check for increasing order, strict or non-strict
@@ -76,7 +82,7 @@ SEXP do_is_ordered (SEXP x, SEXP increasing, SEXP strictly)
   */
   int_x = INTEGER(x);
   if(LOGICAL(increasing)[ 0 ] == 1) { /* INCREASING */
-    /* Not increasing order if first element is NA */
+    /* Not increasing order if first element is NA.  We know x has at least 1 element. */
     if( int_x[0] == NA_INTEGER )
       return ScalarLogical(0);