conf fuzzer: also check for an empty file

Reviewed-by: NTim Hudson <tjh@openssl.org>
Reviewed-by: NRich Salz <rsalz@openssl.org>

GH: #1828

fuzz/conf.c

@@ -20,10 +20,15 @@ int FuzzerInitialize(int *argc, char ***argv) {
 }
 
 int FuzzerTestOneInput(const uint8_t *buf, size_t len) {
-    CONF *conf = NCONF_new(NULL);
-    BIO *in = BIO_new(BIO_s_mem());
+    CONF *conf;
+    BIO *in;
     long eline;
 
+    if (len == 0)
+        return 0;
+
+    conf = NCONF_new(NULL);
+    in = BIO_new(BIO_s_mem());
     OPENSSL_assert((size_t)BIO_write(in, buf, len) == len);
     NCONF_load_bio(conf, in, &eline);
     NCONF_free(conf);