title: Error when extracting values from a rasterBrick or rasterStack
body: <p>I am having trouble extracting values or a point from a multi band raster of class <code>rasterStack</code> or <code>rasterBrick</code>. 'extract' works well with the individual rasters but posts an error when applied to the <code>rasterStack</code> or <code>brick</code>.  </p>
  
  <pre><code>&gt; all.var
  class       : RasterBrick 
  dimensions  : 89, 180, 16020, 34  (nrow, ncol, ncell, nlayers)
  resolution  : 2, 2  (x, y)
  extent      : -179, 181, -89, 89  (xmin, xmax, ymin, ymax)
  coord. ref. : +proj=longlat +datum=WGS84 +no_defs +ellps=WGS84 +towgs84=0,0,0 
  data source : in memory
  names       :    period_max, pct_signif_periodmax, pct_coi_periodmax,    pct_ispos_signif, events_pos_periodmax, events_neg_periodmax, events_pos_all, events_neg_all,  maxpower_pos,  maxpower_neg, maxpower_events_pos, maxpower_events_neg, maxpower_pos_norm, maxpower_neg_norm, maxpower_events_pos_norm, ... 
  
  &gt; point
  Lon      Lat
  1  166.2790 -10.2690
  2   26.9000 -33.6000
  3  153.6209 -28.7001
  4  113.8333 -28.6833
  5  153.6335 -28.6591
  6  153.5836 -28.4643
  7   32.6833 -27.5333
  8   32.6880 -27.5260
  9   32.6880 -27.5260
  10  32.6880 -27.5260
  
  
  &gt; point.extract&lt;-extract(all.var, point, buffer=50000,na.rm=TRUE,fun=mean)
  Error in apply(x, 2, fun2) : dim(X) must have a positive length
  </code></pre>
  
  <p>This works with individual rasters but fails with stack/brick and elicits an error only when  I use a buffer argument.</p>