Add missing check for valid i and j

For some reason, an xts object containing logical data with a real
(numeric) index did not receive the same treatment as all other
data/index types.

All other branches check that:
* i is not NA
* i is not > nrow(x)
* j is not > ncol(x)

Add the same check to this branch of logic.

Fixes #163.

src/subset.c

@@ -136,6 +136,10 @@ SEXP _do_subset_xts (SEXP x, SEXP sr, SEXP sc, SEXP drop) {
       real_nindex = REAL(nindex);
       real_oindex = REAL(oindex);
       for(i=0; i<nr; i++) {
+        if(int_sr[i] == NA_INTEGER)
+          error("'i' contains NA");
+        if(int_sr[i] > nrs || int_sc[j] > ncs)
+          error("'i' or 'j' out of range");
         real_nindex[i] = real_oindex[int_sr[i]-1];
         if(int_sc[j] == NA_INTEGER)
           int_result[i+j*nr] = NA_INTEGER;