Don't malloc(0) on empty extensions list.

Caught by clang scan-build.

Change-Id: I29092d659f1ac21c6a74b925f6abc0283e7652fc
Reviewed-on: https://boringssl-review.googlesource.com/1344
Reviewed-by: Adam Langley <agl@google.com>

ssl/t1_lib.c

@@ -255,6 +255,11 @@ static int tls1_check_duplicate_extensions(const CBS *cbs)
 		num_extensions++;
 		}
 
+	if (num_extensions == 0)
+		{
+		return 1;
+		}
+
 	extension_types = (uint16_t*)OPENSSL_malloc(sizeof(uint16_t) * num_extensions);
 	if (extension_types == NULL)
 		{