Don't leak in err_set_error_data if error queue is empty.

Caught by clang scan-build.

Change-Id: I16496bfc469e809348199057adfb24c33c426ccb
Reviewed-on: https://boringssl-review.googlesource.com/2207
Reviewed-by: Adam Langley <agl@google.com>

crypto/err/err.c

@@ -474,6 +474,9 @@ static void err_set_error_data(char *data, int flags) {
   struct err_error_st *error;
 
   if (state->top == state->bottom) {
+    if (flags & ERR_FLAG_MALLOCED) {
+      OPENSSL_free(data);
+    }
     return;
   }