conf-parser: fix wrong argument given to log_syntax_invalid_utf8

The condition is on "word", hence we give word instead of rvalue.

An assert would be triggered if !utf8_is_valid(word) is true and
rvalue == NULL, since log_syntax_invalid_utf8 calls utf8_escape_invalid
which calls assert(str).

A test case has been added to test with valid and invalid utf8.

src/shared/conf-parser.c

@@ -793,7 +793,7 @@ int config_parse_strv(const char *unit,
                 }
 
                 if (!utf8_is_valid(word)) {
-                        log_syntax_invalid_utf8(unit, LOG_ERR, filename, line, rvalue);
+                        log_syntax_invalid_utf8(unit, LOG_ERR, filename, line, word);
                         free(word);
                         continue;
                 }

src/test/test-conf-parser.c

@@ -180,6 +180,8 @@ static void test_config_parse_strv(void) {
         test_config_parse_strv_one("foo", STRV_MAKE("foo"));
         test_config_parse_strv_one("foo bar foo", STRV_MAKE("foo", "bar", "foo"));
         test_config_parse_strv_one("\"foo bar\" foo", STRV_MAKE("foo bar", "foo"));
+        test_config_parse_strv_one("\xc3\x80", STRV_MAKE("\xc3\x80"));
+        test_config_parse_strv_one("\xc3\x7f", STRV_MAKE_EMPTY);
 }
 
 static void test_config_parse_mode(void) {