Fix some unchecked mallocs.

BUG=456599

Change-Id: Id0652c2aff1cb8a5de35350feb8410285b3fef20
Reviewed-on: https://boringssl-review.googlesource.com/3330
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/ssl/ssl_ciph.c b/ssl/ssl_ciph.c
index 60b9747..e792533 100644
--- a/ssl/ssl_ciph.c
+++ b/ssl/ssl_ciph.c
@@ -1262,8 +1262,9 @@
   if (buf == NULL) {
     len = 128;
     buf = OPENSSL_malloc(len);
-    if (buf == NULL)
-      return "OPENSSL_malloc Error";
+    if (buf == NULL) {
+      return NULL;
+    }
   } else if (len < 128) {
     return "Buffer too small";
   }