Ignore empty files when decrypting with Widevine

This happens because the input file has no actual useful data, likely just a bunch of headers and nothing of use. Therefore shaka skips and returns OK, yet didnt make any file at the decrypted path.

This fixes a crash when it tries to move the non-existent decrypted file to the input file location, causing the rest of the download to halt.

devine/core/drm/widevine.py

@@ -258,11 +258,16 @@ class Widevine:
                 universal_newlines=True
             )
 
+            stream_skipped = False
+
             shaka_log_buffer = ""
             for line in iter(p.stderr.readline, ""):
                 line = line.strip()
                 if not line:
                     continue
+                if "Skip stream" in line:
+                    # file/segment was so small that it didn't have any actual data, ignore
+                    stream_skipped = True
                 if ":INFO:" in line:
                     continue
                 if "Insufficient bits in bitstream for given AVC profile" in line:
@@ -283,14 +288,15 @@ class Widevine:
 
             if p.returncode != 0:
                 raise subprocess.CalledProcessError(p.returncode, arguments)
+
+            path.unlink()
+            if not stream_skipped:
+                shutil.move(decrypted_path, path)
         except subprocess.CalledProcessError as e:
             if e.returncode == 0xC000013A:  # STATUS_CONTROL_C_EXIT
                 raise KeyboardInterrupt()
             raise
 
-        path.unlink()
-        shutil.move(decrypted_path, path)
-
     class Exceptions:
         class PSSHNotFound(Exception):
             """PSSH (Protection System Specific Header) was not found."""