decodetree: Ensure build_tree does not include values outside insnmask

Reproduced with "scripts/decodetree.py /dev/null".
Reviewed-by: NBastian Koppelmann <kbastian@mail.uni-paderborn.de>
Reviewed-by: NEduardo Habkost <ehabkost@redhat.com>
Signed-off-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: NRichard Henderson <richard.henderson@linaro.org>

scripts/decodetree.py

@@ -784,7 +784,7 @@ class Tree:
 
 def build_tree(pats, outerbits, outermask):
     # Find the intersection of all remaining fixedmask.
-    innermask = ~outermask
+    innermask = ~outermask & insnmask
     for i in pats:
         innermask &= i.fixedmask