BaseTools/GenerateCapsule.py: Require --output for --decode

--decode unconditionally uses args.OutputFile.name as a prefix for
output files that it creates and fails in a non-pretty way without
--output option.

This doesn't address creation/truncation of the file specified via
--output, but at least you're able to decode a capsule.

Signed-off-by: Sergii Dmytruk <sergii.dmytruk@3mdeb.com>

BaseTools/Source/Python/Capsule/GenerateCapsule.py

@@ -519,6 +519,10 @@ def Validate(self, args):
                     else:
                         raise argparse.ArgumentTypeError ('--update-image-index must be an integer in range 0x1..0xff')
 
+            if args.Decode:
+                if args.OutputFile is None:
+                    raise argparse.ArgumentTypeError ('--decode requires --output')
+
             if self.UseSignTool:
                 if self.SignToolPfxFile is not None:
                     self.SignToolPfxFile.close()