ansi: return NIL for file-string-length if can't determine it.

Previously we returned error for streams which are not file streams. We could
try to loose this test for ECL_ANSI_STREAM_P, but that would require confirming,
that all ECL's ANSI streams have string encoders defined.

src/c/file.d

@@ -4812,7 +4812,8 @@ cl_file_string_length(cl_object stream, cl_object string)
     }
   }
   unlikely_if (!ECL_FILE_STREAM_P(stream)) {
-    not_a_file_stream(stream);
+    /* Spec says, that if cannot be determined we return NIL. */
+    return ECL_NIL;
   }
   switch (ecl_t_of(string)) {
 #ifdef ECL_UNICODE