coverity - fix tainted

client/emv/emv_pk.c

@@ -408,7 +408,8 @@ static struct emv_pk *emv_pk_get_ca_pk_from_file(const char *fname,
         char buf[2048];
         if (fgets(buf, sizeof(buf), f) == NULL)
             break;
-
+        if (buf == NULL)
+            break;
         struct emv_pk *pk = emv_pk_parse_pk(buf);
         if (!pk)
             continue;