workaround for num_cpus returning 0 (#1034)

workaround for num_cpus returning 0

src/output/details.rs

@@ -147,7 +147,11 @@ impl<'a> AsRef<File<'a>> for Egg<'a> {
 
 impl<'a> Render<'a> {
     pub fn render<W: Write>(mut self, w: &mut W) -> io::Result<()> {
-        let mut pool = Pool::new(num_cpus::get() as u32);
+        let n_cpus = match num_cpus::get() as u32 {
+            0 => 1,
+            n => n,
+        };
+        let mut pool = Pool::new(n_cpus);
         let mut rows = Vec::new();
 
         if let Some(ref table) = self.opts.table {