decodetree: Diagnose empty pattern group

Test err_pattern_group_empty.decode failed with exception:

Traceback (most recent call last):
  File "./scripts/decodetree.py", line 1424, in <module> main()
  File "./scripts/decodetree.py", line 1342, in main toppat.build_tree()
  File "./scripts/decodetree.py", line 627, in build_tree
    self.tree = self.__build_tree(self.pats, self.fixedbits,
  File "./scripts/decodetree.py", line 607, in __build_tree
    fb = i.fixedbits & innermask
TypeError: unsupported operand type(s) for &: 'NoneType' and 'int'

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

scripts/decodetree.py

@@ -506,6 +506,12 @@ def output_code(self, i, extracted, outerbits, outermask):
                 output(ind, '}\n')
             else:
                 p.output_code(i, extracted, p.fixedbits, p.fixedmask)
+
+    def build_tree(self):
+        if not self.pats:
+            error_with_file(self.file, self.lineno, 'empty pattern group')
+        super().build_tree()
+
 #end IncMultiPattern