Fix OSSL_STORE's 'file' loader: make sure peekbuf is initialised

This quiets down complaints about the use of uninitialised memory

[extended tests]
Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4340)

crypto/store/loader_file.c

@@ -855,7 +855,7 @@ static OSSL_STORE_LOADER_CTX *file_open(const OSSL_STORE_LOADER *loader,
         }
     } else {
         BIO *buff = NULL;
-        char peekbuf[4096];
+        char peekbuf[4096] = { 0, };
 
         if ((buff = BIO_new(BIO_f_buffer())) == NULL
             || (ctx->_.file.file = BIO_new_file(path, "rb")) == NULL) {