Check seteuid result in quota wrapper

Handle failures from seteuid(0) in su_quotactl() before calling
quotactl(). This fixes the warn_unused_result compiler warning and
returns an error to the quota caller if privilege elevation fails.

src/nwvolume.c

@@ -953,8 +953,14 @@ static int su_quotactl(int cmd, const char * special, int id, caddr_t addr)
 {
   int result;
   int euid=geteuid();
-  seteuid(0);
+
+  if (seteuid(0)) {
+    errorp(1, "seteuid", "cannot change to uid=0\n");
+    return(-1);
+  }
+
   result=quotactl(cmd, special, id, addr);
+
   if (seteuid(euid)) {
     errorp(1, "seteuid", "cannot change to uid=%d\n", euid);
     exit(1);