mctp-netlink: Don't qsort() when target list is NULL

Mitigate the following ubsan splat:

    Feb 27 09:32:23 test mctpd[1034]: ../git/src/mctp-netlink.c:960:3: runtime error: null pointer passed as argument 1, which is declared to never be null
    Feb 27 09:32:23 test mctpd[1034]:     #0 0x4552c4  (/usr/sbin/mctpd+0x4a2c4) (BuildId: 0a2c71201a0ddde8f9bbe7d2fd65628c4e08f5d8)
    Feb 27 09:32:23 test mctpd[1034]:     #1 0x42c3b0  (/usr/sbin/mctpd+0x213b0) (BuildId: 0a2c71201a0ddde8f9bbe7d2fd65628c4e08f5d8)
    Feb 27 09:32:23 test mctpd[1034]:     #2 0xa623b4ac  (/usr/lib/libc.so.6+0x1f4ac) (BuildId: f7dfc12cfaed3ca290b3c7f41ef9145c0de0fe6b)
    Feb 27 09:32:23 test mctpd[1034]:     #3 0xa623b598 in __libc_start_main (/usr/lib/libc.so.6+0x1f598) (BuildId: f7dfc12cfaed3ca290b3c7f41ef9145c0de0fe6b)

glibc (e.g. 2.41) declares qsort as:

    extern void qsort (void *__base, size_t __nmemb, size_t __size,
    		   __compar_fn_t __compar) __nonnull ((1, 4));

Signed-off-by: Andrew Jeffery <andrew@codeconstruct.com.au>

Author: amboar

src/mctp-netlink.c

@@ -952,13 +952,17 @@ static void sort_linkmap(mctp_nl *nl)
 {
 	size_t i;
 
-	qsort(nl->linkmap, nl->linkmap_count, sizeof(*nl->linkmap),
-	      cmp_ifindex);
+	if (nl->linkmap) {
+		qsort(nl->linkmap, nl->linkmap_count, sizeof(*nl->linkmap),
+		      cmp_ifindex);
+	}
 
 	for (i = 0; i < nl->linkmap_count; i++) {
 		struct linkmap_entry *entry = &nl->linkmap[i];
-		qsort(entry->local_eids, entry->num_local, sizeof(mctp_eid_t),
-		      cmp_eid);
+		if (entry->local_eids) {
+			qsort(entry->local_eids, entry->num_local,
+			      sizeof(mctp_eid_t), cmp_eid);
+		}
 	}
 }