PSSH: Fix get_as_box parsing on arbitrary init data

An IOError can occur if the mp4 box parsing fails because it could not read enough bytes.

pywidevine/pssh.py

@@ -112,7 +112,7 @@ class PSSH:
         if isinstance(data, bytes):
             try:
                 data = Box.parse(data)
-            except construct.ConstructError:
+            except (IOError, construct.ConstructError):
                 if strict:
                     try:
                         cenc_header = WidevinePsshData()