net/socket: minor patch for inet_pton

1. If there are no dot (".") on given IPv4 address, return 0.

2. Fix return value of inet_pton when received invalid af.
If address family is not neither AF_INET or AF_INET6, inet_pton
should return -1 as Linux man page (man inet_pton).
 - If af does not contain a valid address family, -1 is returned
   and errno is set to EAFNOSUPPORT.

os/net/lwip/src/core/ipv4/ip4_addr.c

@@ -226,6 +226,7 @@ int ip4addr_aton(const char *cp, ip4_addr_t *addr)
 	if (c != '\0' && !isspace(c)) {
 		return 0;
 	}
+
 	/*
 	 * Concoct the address according to
 	 * the number of parts specified.
@@ -236,7 +237,7 @@ int ip4addr_aton(const char *cp, ip4_addr_t *addr)
 		return 0;				/* initial nondigit */
 
 	case 1:					/* a -- 32 bits */
-		break;
+		return 0;
 
 	case 2:					/* a.b -- 8.24 bits */
 		if (val > 0xffffffUL) {

os/net/socket/bsd_socket_api.c

@@ -314,7 +314,9 @@ int inet_pton(int af, FAR const char *src, FAR void *dest)
 		return ip6addr_aton(((const char *)(src)), ((ip6_addr_t*)(dest)));
 	else
 #endif
-		return 0;
+	{
+		return -1;
+	}
 }
 
 #ifdef CONFIG_NET_LWIP_NETDB