Check for duplicate jump tables before performing any IR mutations.

This isn't fixing an actual bug, but it does make `add_jt` more
consistent with the other `add_*` functions.

lib/reader/src/parser.rs

@@ -233,11 +233,12 @@ impl<'a> Context<'a> {
 
     // Allocate a new jump table.
     fn add_jt(&mut self, jt: JumpTable, data: JumpTableData, loc: Location) -> ParseResult<()> {
+        self.map.def_jt(jt, loc)?;
         while self.function.jump_tables.next_key().index() <= jt.index() {
             self.function.create_jump_table(JumpTableData::new());
         }
         self.function.jump_tables[jt] = data;
-        self.map.def_jt(jt, loc)
+        Ok(())
     }
 
     // Resolve a reference to a jump table.