Fix print_supported_protocols and print_supported_protocols_usage

These were broken in 7cb8996, when the
empty sentinel value at the end of the array was removed, without changing
the way these functions iterate over that array.

For some reason, this continues to work on Linux (probably due to `calloc`
allocating more zeroed bytes than we request, in
`openconnect_get_supported_protocols`), but is causing the expected SIGSEGV on
Solaris:
https://lists.infradead.org/pipermail/openconnect-devel/2020-April/005640.html

Fix:

- Modify `print_supported_protocols` and `print_supported_protocols_usage` to
  rely on the length returned by `openconnect_get_supported_protocols`.
- Restore the sentinel value at the end of the array returned by
  `openconnect_get_supported_protocols`, to preserve ABI compatibility for
  other users who may depend on this sentinel.

Signed-off-by: Daniel Lenski <dlenski@gmail.com>

library.c

@@ -195,7 +195,13 @@ int openconnect_get_supported_protocols(struct oc_vpn_proto **protos)
 	struct oc_vpn_proto *pr;
 	int i;
 
-	*protos = pr = calloc(NR_PROTOS, sizeof(*pr));
+        /* The original version of this function included an all-zero
+         * sentinel value at the end of the array, so we must continue
+         * to do so for ABI compatibility even though it's
+         * functionally redundant as a marker of the array's length,
+         * along with the explicit length in the return value.
+         */
+	*protos = pr = calloc(NR_PROTOS + 1, sizeof(*pr));
 	if (!pr)
 		return -ENOMEM;
 

main.c

@@ -659,10 +659,12 @@ static void print_supported_protocols(void)
 {
 	const char *comma = ", ", *sep = comma + 1;
 	struct oc_vpn_proto *protos, *p;
+	int n;
 
-	if (openconnect_get_supported_protocols(&protos)>=0) {
+	n = openconnect_get_supported_protocols(&protos);
+        if (n>=0) {
 		printf(_("Supported protocols:"));
-		for (p=protos; p->name; p++) {
+		for (p=protos; n; p++, n--) {
 			printf("%s%s%s", sep, p->name, p==protos ? _(" (default)") : "");
 			sep = comma;
 		}
@@ -674,10 +676,12 @@ static void print_supported_protocols(void)
 static void print_supported_protocols_usage(void)
 {
 	struct oc_vpn_proto *protos, *p;
+	int n;
 
-	if (openconnect_get_supported_protocols(&protos)>=0) {
+	n = openconnect_get_supported_protocols(&protos);
+        if (n>=0) {
 		printf("\n%s:\n", _("Set VPN protocol"));
-		for (p=protos; p->name; p++)
+		for (p=protos; n; p++, n--)
 			printf("      --protocol=%-16s %s%s\n",
 				   p->name, p->description, p==protos ? _(" (default)") : "");
 		openconnect_free_supported_protocols(protos);