Fix memory leak in decoder test code

Fix memory leak in decoder test code

==187724== 917 bytes in 1 blocks are definitely lost in loss record 3,046 of 3,207
==187724==    at 0x4C2EBAB: malloc (vg_replace_malloc.c:299)
==187724==    by 0x563C435: g_malloc (in /usr/lib64/libglib-2.0.so.0.5600.3)
==187724==    by 0x5655D82: g_strdup (in /usr/lib64/libglib-2.0.so.0.5600.3)
==187724==    by 0x4E43937: py_str_as_str (util.c:371)
==187724==    by 0x4E415E1: srd_decoder_doc_get (decoder.c:892)
==187724==    by 0x40281D: test_doc_get (decoder.c:395)
==187724==    by 0x53E51D5: srunner_run_tagged (in /usr/lib64/libcheck.so.0.0.0)
==187724==    by 0x401237: main (main.c:51)