Add bytearray support back to imdecode (#12855, #12868)

1. Avoid raise exception when input is bytearray.
2. Avoid OpenCV crash for empty input.

python/mxnet/image/image.py

@@ -93,7 +93,7 @@ def imdecode(buf, *args, **kwargs):
 
     Parameters
     ----------
-    buf : str/bytes or numpy.ndarray
+    buf : str/bytes/bytearray or numpy.ndarray
         Binary image data as string or numpy ndarray.
     flag : int, optional, default=1
         1 for three channel color output. 0 for grayscale output.
@@ -135,10 +135,15 @@ def imdecode(buf, *args, **kwargs):
     <NDArray 224x224x3 @cpu(0)>
     """
     if not isinstance(buf, nd.NDArray):
-        if sys.version_info[0] == 3 and not isinstance(buf, (bytes, np.ndarray)):
-            raise ValueError('buf must be of type bytes or numpy.ndarray,'
+        if sys.version_info[0] == 3 and not isinstance(buf, (bytes, bytearray, np.ndarray)):
+            raise ValueError('buf must be of type bytes, bytearray or numpy.ndarray,'
                              'if you would like to input type str, please convert to bytes')
         buf = nd.array(np.frombuffer(buf, dtype=np.uint8), dtype=np.uint8)
+
+    if len(buf) == 0:
+        # empty buf causes OpenCV crash.
+        raise ValueError("input buf cannot be empty.")
+
     return _internal._cvimdecode(buf, *args, **kwargs)