shared, libsystemd-daemon: check for empty strings in strto*l conversions

strtol() and friends may set EINVAL if no conversion was performed, but
they are not required to do so. In practice they don't. We need to check
for it.

https://bugzilla.redhat.com/show_bug.cgi?id=870577

src/shared/conf-parser.c

@@ -865,7 +865,7 @@ int config_parse_mode(
 
         errno = 0;
         l = strtol(rvalue, &x, 8);
-        if (!x || *x || errno) {
+        if (!x || x == rvalue || *x || errno) {
                 log_error("[%s:%u] Failed to parse mode value, ignoring: %s", filename, line, rvalue);
                 return 0;
         }