Check the return value of ASN1_STRING_length

ASN1_STRING_length gets the field 'length' of msg, which
can be manipulated through a crafted input.
Add a check to avoid error execution of OPENSSL_malloc().

CLA: trivial

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from openssl#15583)

crypto/ts/ts_verify_ctx.c

@@ -126,6 +126,8 @@ TS_VERIFY_CTX *TS_REQ_to_TS_VERIFY_CTX(TS_REQ *req, TS_VERIFY_CTX *ctx)
         goto err;
     msg = imprint->hashed_msg;
     ret->imprint_len = ASN1_STRING_length(msg);
+    if (ret->imprint_len <= 0)
+        goto err;
     if ((ret->imprint = OPENSSL_malloc(ret->imprint_len)) == NULL)
         goto err;
     memcpy(ret->imprint, ASN1_STRING_get0_data(msg), ret->imprint_len);