Fix segfaul when encoding with the uescape option

There was a bug that was resetting the extra amount of space needed when
encoding unicode characters with the `uescape` option enabled.

c_src/encoder.c

@@ -274,7 +274,7 @@ enc_string(Encoder* e, ERL_NIF_TERM val)
                     if(uval < 0) {
                         return 0;
                     }
-                    esc_extra = utf8_esc_len(uval);
+                    esc_extra += utf8_esc_len(uval);
                     if(ulen < 0) {
                         return 0;
                     }