pvkkdf: Always reset buflen after clearing the buffer

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/17164)

providers/implementations/kdfs/pvkkdf.c

diff --git a/providers/implementations/kdfs/pvkkdf.c b/providers/implementations/kdfs/pvkkdf.c
index 051c625..e953911 100644
--- a/providers/implementations/kdfs/pvkkdf.c
+++ b/providers/implementations/kdfs/pvkkdf.c
@@ -97,13 +97,15 @@
                              const OSSL_PARAM *p)
 {
     OPENSSL_clear_free(*buffer, *buflen);
+    *buffer = NULL;
+    *buflen = 0;
+
     if (p->data_size == 0) {
         if ((*buffer = OPENSSL_malloc(1)) == NULL) {
             ERR_raise(ERR_LIB_PROV, ERR_R_MALLOC_FAILURE);
             return 0;
         }
     } else if (p->data != NULL) {
-        *buffer = NULL;
         if (!OSSL_PARAM_get_octet_string(p, (void **)buffer, 0, buflen))
             return 0;
     }