verity: warn on null-valued keys

BUG=chromium-os:20089
TESTED_ON=Kaen
TEST=Adhoc
'verity mode' should emit "missing value: mode" before the standard usage
message now. So should 'verity mode='.

Change-Id: Ica495d07a84fc1924bea37385cc2439032e5ad66
Signed-off-by: Elly Jones <ellyjones@chromium.org>
Reviewed-on: http://gerrit.chromium.org/gerrit/7346
Reviewed-by: David James <davidjames@chromium.org>
diff --git a/verity_main.cc b/verity_main.cc
index 15fd0aa..8a5b98a 100644
--- a/verity_main.cc
+++ b/verity_main.cc
@@ -60,8 +60,13 @@
 
   for (i = 1; i < argc; i++) {
     splitarg(argv[i], &key, &val);
-    if (!key || !val)
+    if (!key)
       continue;
+    if (!val) {
+      fprintf(stderr, "missing value: %s\n", key);
+      print_usage(argv[0]);
+      return -1;
+    }
     if (!strcmp(key, "alg"))
       alg = val;
     else if (!strcmp(key, "payload"))