Prevent invalid iterator dereference

gcc/rust/ChangeLog:

	* lex/rust-lex.cc
	(Lexer::classify_keyword): Check if iterator is valid before dereferencing.

Signed-off-by: Owen Avery <[email protected]>

Author: powerboat9

gcc/rust/lex/rust-lex.cc

@@ -255,11 +255,12 @@ TokenId
 Lexer::classify_keyword (const std::string &str)
 {
   auto keyword = keywords.find (str);
-  auto id = keyword->second;
 
   if (keyword == keywords.end ())
     return IDENTIFIER;
 
+  auto id = keyword->second;
+
   // We now have the expected token ID of the reserved keyword. However, some
   // keywords are reserved starting in certain editions. For example, `try` is
   // only a reserved keyword in editions >=2018. The language might gain new