qapi: Fix to reject stray 't', 'f' and 'n'

Screwed up in commit e53188a.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>

scripts/qapi.py

@@ -217,17 +217,15 @@ class QAPISchema:
                         return
                     else:
                         string += ch
-            elif self.tok in "tfn":
-                val = self.src[self.cursor - 1:]
-                if val.startswith("true"):
+            elif self.src.startswith("true", self.pos):
                 self.val = True
                 self.cursor += 3
                 return
-                elif val.startswith("false"):
+            elif self.src.startswith("false", self.pos):
                 self.val = False
                 self.cursor += 4
                 return
-                elif val.startswith("null"):
+            elif self.src.startswith("null", self.pos):
                 self.val = None
                 self.cursor += 3
                 return