Fix file pointer leak in utf8_load_text_file

[norihiro]

GCC's static analyzer found a file pointer leak. I moved fclose to the outside of an if closure so that the file pointer will be always released when the file was successfully opened.

Tested with gcc (GCC) 12.2.1 20221121 (Red Hat 12.2.1-4). I configured with option -fanalyzer -Wanalyzer-use-after-free -Wanalyzer-double-free -Wanalyzer-malloc-leak and built the code.