mount: fix potential bad memory access when /proc/self/mountinfo is empty

It's unlikely this can ever be triggered, but let's be safe rather than
sorry, and handle the case where the list of mount points is zero, and
the "l" array thus NULL. let's ensure we allocate at least one entry.

src/mount/mount-tool.c

@@ -736,13 +736,13 @@ static int find_mount_points(const char *what, char ***list) {
                 if (!GREEDY_REALLOC(l, bufsize, n + 2))
                         return log_oom();
 
-                l[n] = strdup(where);
-                if (!l[n])
-                        return log_oom();
-
-                n++;
+                l[n++] = where;
+                where = NULL;
         }
 
+        if (!GREEDY_REALLOC(l, bufsize, n + 1))
+                return log_oom();
+
         l[n] = NULL;
         *list = l;
         l = NULL; /* avoid freeing */