[gcc r14-7675] gccrs: Prevent invalid iterator dereference

[gcc r14-7675] gccrs: Prevent invalid iterator dereference

[Arthur Cohen]

https://gcc.gnu.org/g:45c01fae2a2afb30b0ca933e9d7b807acb167d23

commit r14-7675-g45c01fae2a2afb30b0ca933e9d7b807acb167d23
Author: Owen Avery <powerboat9.gamer@gmail.com>
Date:   Sat Jun 17 00:32:38 2023 -0400

    gccrs: 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 <powerboat9.gamer@gmail.com>

Diff:
---
 gcc/rust/lex/rust-lex.cc | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/gcc/rust/lex/rust-lex.cc b/gcc/rust/lex/rust-lex.cc
index c910c7cf83a..23579e51944 100644
--- a/gcc/rust/lex/rust-lex.cc
+++ b/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