Fix memory leaks related to GooString usage
Could be triggered with valgrind --leak-check=full ./utils/pdftoppm clusterfuzz-testcase-minimized-gdal_fuzzer-4924367240298496 -o out
clusterfuzz-testcase-minimized-gdal_fuzzer-4924367240298496
Could be triggered with valgrind --leak-check=full ./utils/pdftoppm clusterfuzz-testcase-minimized-gdal_fuzzer-4924367240298496 -o out
clusterfuzz-testcase-minimized-gdal_fuzzer-4924367240298496