base-files: ipcalc.sh: fail when network is too small

It's possible to move range boundaries in a way that the start address
lies behind the end address.
Detect this condition and exit with an error message.

Signed-off-by: Leon M. George <leon@georgemail.eu>

package/base-files/files/bin/ipcalc.sh

@@ -44,13 +44,14 @@ BEGIN {
 	}
 
 	network=and(ipaddr,netmask)
+	prefix=32-bitcount(compl32(netmask))
 	broadcast=or(network,compl32(netmask))
 
 	print "IP="int2ip(ipaddr)
 	print "NETMASK="int2ip(netmask)
 	print "BROADCAST="int2ip(broadcast)
 	print "NETWORK="int2ip(network)
-	print "PREFIX="32-bitcount(compl32(netmask))
+	print "PREFIX="prefix
 
 	# range calculations:
 	# ipcalc <ip> <netmask> <start> <num>
@@ -68,6 +69,11 @@ BEGIN {
 	if (end>limit) end=limit
 	if (end==ipaddr) end=ipaddr-1
 
+	if (start>end) {
+		print "network ("int2ip(network)"/"prefix") too small" > "/dev/stderr"
+		exit(1)
+	}
+
 	if (ipaddr > start && ipaddr < end) {
 		print "ipaddr inside range" > "/dev/stderr"
 		exit(1)