Fix dual-callback case (#3537)

This fixes a bug where using the callback version of createPresignedPost
without proper AWS credentials configured would result in an
un-catchable error and crash the node process.

It is not possible to write a test to capture this error due to the
nature of the error.  The callback is called, and only after that does
the crash occur.

.changes/next-release/bugfix-S3-9d19dd09.json

@@ -0,0 +1,5 @@
+{
+  "type": "bugfix",
+  "category": "S3",
+  "description": "fixed a bug where createPresignedPost could result in a process crash."
+}

lib/services/s3.js

@@ -1079,9 +1079,13 @@ AWS.util.update(AWS.S3.prototype, {
       config.getCredentials(function (err) {
         if (err) {
           callback(err);
+        } else {
+          try {
+            callback(null, finalizePost());
+          } catch (err) {
+            callback(err);
+          }
         }
-
-        callback(null, finalizePost());
       });
     } else {
       return finalizePost();