fix(put): don't flush if an error happened

What happens now is that the index is written even if there was an error
in the content. We don't want this.

lib/put.js

@@ -37,6 +37,7 @@ function putStream (cache, key, opts = {}) {
   opts = putOpts(opts)
   let integrity
   let size
+  let error
 
   let memoData
   const pipeline = new Pipeline()
@@ -58,28 +59,33 @@ function putStream (cache, key, opts = {}) {
     .on('size', (s) => {
       size = s
     })
+    .on('error', (err) => {
+      error = err
+    })
 
   pipeline.push(contentStream)
 
   // last but not least, we write the index and emit hash and size,
   // and memoize if we're doing that
   pipeline.push(new Flush({
     flush () {
-      return index
-        .insert(cache, key, integrity, { ...opts, size })
-        .then((entry) => {
-          if (memoize && memoData) {
-            memo.put(cache, entry, memoData, opts)
-          }
+      if (!error) {
+        return index
+          .insert(cache, key, integrity, { ...opts, size })
+          .then((entry) => {
+            if (memoize && memoData) {
+              memo.put(cache, entry, memoData, opts)
+            }
 
-          if (integrity) {
-            pipeline.emit('integrity', integrity)
-          }
+            if (integrity) {
+              pipeline.emit('integrity', integrity)
+            }
 
-          if (size) {
-            pipeline.emit('size', size)
-          }
-        })
+            if (size) {
+              pipeline.emit('size', size)
+            }
+          })
+      }
     },
   }))