Replace size_t with int and add the check for the EVP_MD_get_size()

Replace the type of "digest_size" with int to avoid implicit conversion when it is assigned by EVP_MD_get_size().
Moreover, add the check for the "digest_size".

Fixes: 29ce1066bc ("Update the demos/README file because it is really old. New demos should provide best practice for API use. Add demonstration for computing a SHA3-512 digest - digest/EVP_MD_demo")
Signed-off-by: Jiasheng Jiang <jiasheng@purdue.edu>

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Kurt Roeckx <kurt@roeckx.be>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/23924)

demos/digest/BIO_f_md.c

@@ -42,7 +42,7 @@ int main(int argc, char *argv[])
     BIO *bio_digest = NULL, *reading = NULL;
     EVP_MD *md = NULL;
     unsigned char buffer[512];
-    size_t digest_size;
+    int digest_size;
     char *digest_value = NULL;
     int j;
 
@@ -68,6 +68,11 @@ int main(int argc, char *argv[])
         goto cleanup;
     }
     digest_size = EVP_MD_get_size(md);
+    if (digest_size <= 0) {
+        fprintf(stderr, "EVP_MD_get_size returned invalid size.\n");
+        goto cleanup;
+    }
+
     digest_value = OPENSSL_malloc(digest_size);
     if (digest_value == NULL) {
         fprintf(stderr, "Can't allocate %lu bytes for the digest value.\n", (unsigned long)digest_size);