fix c-warnings; fix CI

Author: tamird

ext/mysql2/client.c

@@ -129,7 +129,6 @@ static VALUE rb_raise_mysql2_error(mysql_client_wrapper *wrapper) {
                  UINT2NUM(mysql_errno(wrapper->client)),
                  rb_sql_state);
   rb_exc_raise(e);
-  return Qnil;
 }
 
 static void *nogvl_init(void *ptr) {
@@ -525,8 +524,6 @@ static VALUE disconnect_and_raise(VALUE self, VALUE error) {
   }
 
   rb_exc_raise(error);
-
-  return Qnil;
 }
 
 static VALUE do_query(void *args) {

ext/mysql2/mysql2_ext.h

@@ -29,8 +29,10 @@ void Init_mysql2(void);
 #endif
 
 #if defined(__GNUC__) && (__GNUC__ >= 3)
+#define RB_MYSQL_NORETURN __attribute__ ((noreturn))
 #define RB_MYSQL_UNUSED __attribute__ ((unused))
 #else
+#define RB_MYSQL_NORETURN
 #define RB_MYSQL_UNUSED
 #endif
 

ext/mysql2/statement.h

@@ -13,6 +13,6 @@ void init_mysql2_statement(void);
 void decr_mysql2_stmt(mysql_stmt_wrapper *stmt_wrapper);
 
 VALUE rb_mysql_stmt_new(VALUE rb_client, VALUE sql);
-void rb_raise_mysql2_stmt_error(mysql_stmt_wrapper *stmt_wrapper);
+void rb_raise_mysql2_stmt_error(mysql_stmt_wrapper *stmt_wrapper) RB_MYSQL_NORETURN;
 
 #endif