Avoid dangling ptrs in header and data params for PEM_read_bio_ex

In the event of a failure in PEM_read_bio_ex() we free the buffers we
allocated for the header and data buffers. However we were not clearing
the ptrs stored in *header and *data. Since, on success, the caller is
responsible for freeing these ptrs this can potentially lead to a double
free if the caller frees them even on failure.

Thanks to Dawei Wang for reporting this issue.

Based on a proposed patch by Kurt Roeckx.

CVE-2022-4450
Reviewed-by: NPaul Dale <pauli@openssl.org>
Reviewed-by: NHugo Landau <hlandau@openssl.org>
Signed-off-by: Ncode4lala <fengziteng2@huawei.com>

crypto/pem/pem_lib.c

@@ -940,7 +940,9 @@ int PEM_read_bio_ex(BIO *bp, char **name_out, char **header,
     *data = pem_malloc(len, flags);
     if (*header == NULL || *data == NULL) {
         pem_free(*header, flags, 0);
+        *header = NULL;
         pem_free(*data, flags, 0);
+        *data = NULL;
         goto end;
     }
     BIO_read(headerB, *header, headerlen);