yylex: use grub_fatal for exit.

lexer calls yylex_fatal on fatal internal errors. yylex_fatal itself is
declared as noreturn and calls exit. Returning from noreturn function has
unpredictable consequences.

grub-core/script/yylex.l

@@ -92,7 +92,7 @@ typedef size_t yy_size_t;
 #define stdout 0
 
 #define fprintf(...) 0
-#define exit(...)
+#define exit(...) grub_fatal("fatal error in lexer")
 #endif
 
 }