udhcpc: do not use inet_addr() to parse -r REQ_IP, it's deprecated

...and we did not error-check it, and this is the only use of it:

function                                             old     new   delta
inet_addr                                             37       -     -37

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/networking/udhcp/dhcpc.c b/networking/udhcp/dhcpc.c
index d67f60c..ab9edd2 100644
--- a/networking/udhcp/dhcpc.c
+++ b/networking/udhcp/dhcpc.c
@@ -1284,7 +1284,8 @@
 		memcpy(p + OPT_DATA + 3, str_F, len); /* do not store NUL byte */
 	}
 	if (opt & OPT_r)
-		requested_ip = inet_addr(str_r);
+		if (!inet_aton(str_r, (void*)&requested_ip))
+			bb_show_usage();
 #if ENABLE_FEATURE_UDHCP_PORT
 	if (opt & OPT_P) {
 		CLIENT_PORT = xatou16(str_P);