MT#61856 call_flags: improve styling in `call_ng_flags_prefix()`

The nested if-statement is correctly written,
but visually is fragile. Simple curly braces will prevent
anyone in the future introducing a dangling-if style mistake.

Change-Id: I3632702390f046c317eac4e5fef76bc7db707081

daemon/call_flags.c

@@ -648,9 +648,10 @@ INLINE int call_ng_flags_prefix(str *s_ori, const char *prefix,
 {
 	size_t len = strlen(prefix);
 	str s = *s_ori;
-	if (len > 0)
+	if (len > 0) {
 		if (str_shift_cmp(&s, prefix))
 			return 0;
+	}
 	cb(&s, 0, arg);
 	return 1;
 }