Don't invoke .ready() callback twice if it throws

Previously, when the callback passed to `.ready()` would throw,
it would be called again because it was also registered as a
`.catch()` handler for the same Promise that invoked it.

index.js

@@ -108,7 +108,7 @@ Blake2b.ready = function (cb) {
     rdy = WebAssembly.instantiate(buf).then(setup)
   }
 
-  return rdy.then(cb).catch(cb)
+  return rdy.then(cb, cb)
 }
 
 function noop () {}

test.js

@@ -70,3 +70,16 @@ blake2b.ready(function () {
     })
   })
 })
+
+tape('.ready()', function (t) {
+  var invokeCount = 0;
+  blake2b
+    .ready(function () {
+      invokeCount++
+      throw new Error()
+    })
+    .catch(function () {
+      t.same(invokeCount, 1)
+      t.end()
+    })
+});