Fix coverity 1498607: uninitialised value

Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/17897)

crypto/ec/ecp_nistz256.c

diff --git a/crypto/ec/ecp_nistz256.c b/crypto/ec/ecp_nistz256.c
index ccd6895..77a6738 100644
--- a/crypto/ec/ecp_nistz256.c
+++ b/crypto/ec/ecp_nistz256.c
@@ -978,6 +978,7 @@
         return 0;
     }
 
+    memset(&p, 0, sizeof(p));
     BN_CTX_start(ctx);
 
     if (scalar) {