Handle ISO8601 string outside valid index range

Passing an invalid date/time string to .parseISO8601() when both
'start' and 'end' arguments are also specified caused the function
return the first and last index values.

Now the function returns a first and last value equal to NA_real_,
with a warning. .parseISO8601() is primarily used in `[.xts` where
the returned NA values will cause the subset call to return no
instead of all rows.

Fixes #96.

R/parse8601.R

@@ -133,6 +133,10 @@
    if(is.na(e))
      e <- as.POSIXlt(do.call(lastof,  parse.side(intervals[2])))
  }
+ if(is.na(s) && is.na(e) && !nzchar(DURATION)) {
+   warning("cannot determine first and last time from ", x)
+   return(list(first.time=NA_real_,last.time=NA_real_))
+ }
  if(!missing(start)) {
    start <- as.numeric(start)
    #s <- as.POSIXlt(structure(max(start, as.numeric(s), na.rm=TRUE),

inst/unitTests/runit.subset.R

@@ -192,3 +192,10 @@ test.i_POSIXlt <- function() {
     checkIdentical(y, x[i, ])
   }
 }
+
+# invalid date/time
+test.i_invalid_date_string <- function() {
+  x <- xts(1:10, as.Date("2015-02-20")+0:9)
+  y <- x["2012-02-30"]
+  checkIdentical(y, x[NA,])
+}