tests: Fix possible NULL derefs in virErrorTestMsgs

Add guards to avoid calling strchr when @err_noinfo == NULL or
calling virErrorTestMsgFormatInfoOne when @err_info == NULL as
both would fail with a NULL deref.
Signed-off-by: NJohn Ferlan <jferlan@redhat.com>
ACKed-by: NPeter Krempa <pkrempa@redhat.com>

tests/virerrortest.c

@@ -74,13 +74,13 @@ virErrorTestMsgs(const void *opaque ATTRIBUTE_UNUSED)
             ret = -1;
         }
 
-        if (strchr(err_noinfo, '%')) {
+        if (err_noinfo && strchr(err_noinfo, '%')) {
             VIR_TEST_VERBOSE("\nerror message id %zu contains formatting characters: '%s'\n",
                              i, err_noinfo);
             ret = -1;
         }
 
-        if (virErrorTestMsgFormatInfoOne(err_info) < 0)
+        if (err_info && virErrorTestMsgFormatInfoOne(err_info) < 0)
             ret = -1;
     }