fix possible connection crashes as per PR mscdex#564

see PR mscdex#564 in original repo:
mscdex#546

These crashes might result from user errors... but still it makes sense to not just crash because of accessing uninitialized variables.

Author: Garfonso

lib/Connection.js

@@ -190,8 +190,16 @@ class Connection extends EventEmitter {
       self._resTagged(info)
     })
     parser.on('body', function (stream, info) {
-      let msg = self._curReq.fetchCache[info.seqno]
-      let toget
+      if (!self._curReq) {
+        stream.resume();
+        return;
+      }
+      let msg = undefined;
+      let toget;
+      if (self._curReq.fetchCache.hasOwnProperty(info.seqno)) {
+        msg = self._curReq.fetchCache[info.seqno];
+      }
+
       if (msg === undefined) {
         msg = self._curReq.fetchCache[info.seqno] = {
           msgEmitter: new EventEmitter(),
@@ -402,11 +410,13 @@ class Connection extends EventEmitter {
         self._box = undefined
         cb(err)
       }
-      else {
+      else if (self._box) {
         self._box.name = name
         cb(err, self._box)
+      } else {
+        cb(new Error('No mailbox is currently selected'));
       }
-    })
+    });
   }
   closeBox(shouldExpunge, cb) {
     if (this._box === undefined)
@@ -1183,7 +1193,7 @@ class Connection extends EventEmitter {
       this._curReq.cbargs.push(box)
     }
     else if (type === 'fetch') {
-      if (/^(?:UID )?FETCH/.test(this._curReq.fullcmd)) {
+      if (this._curReq && /^(?:UID )?FETCH/.test(this._curReq.fullcmd)) {
         // FETCH response sent as result of FETCH request
         const msg = this._curReq.fetchCache[info.num], keys = Object.keys(info.text), keyslen = keys.length
         let toget, msgEmitter, j