Code hygiene; initialize some pointers.

CLA: trivial
Reviewed-by: NAndy Polyakov <appro@openssl.org>
Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4515)

crypto/cms/cms_sd.c

@@ -635,7 +635,7 @@ int cms_SignedData_final(CMS_ContentInfo *cms, BIO *chain)
 int CMS_SignerInfo_sign(CMS_SignerInfo *si)
 {
     EVP_MD_CTX *mctx = si->mctx;
-    EVP_PKEY_CTX *pctx;
+    EVP_PKEY_CTX *pctx = NULL;
     unsigned char *abuf = NULL;
     int alen;
     size_t siglen;

crypto/pkcs7/pk7_doit.c

@@ -813,7 +813,7 @@ int PKCS7_dataFinal(PKCS7 *p7, BIO *bio)
 int PKCS7_SIGNER_INFO_sign(PKCS7_SIGNER_INFO *si)
 {
     EVP_MD_CTX *mctx;
-    EVP_PKEY_CTX *pctx;
+    EVP_PKEY_CTX *pctx = NULL;
     unsigned char *abuf = NULL;
     int alen;
     size_t siglen;