memory leak fix: g_variant_print() usage fix

memory leak fix: g_variant_print() usage fix

g_variant_print() allocates memory during call. Save the pointer
so that it can be free'd afterwards.

==10048== 16 bytes in 1 blocks are definitely lost in loss record 17 of 37
==10048==    at 0x4C2DEAE: realloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==10048==    by 0x536C85D: g_realloc (in /lib/x86_64-linux-gnu/libglib-2.0.so.0.4400.1)
==10048==    by 0x53877C6: ??? (in /lib/x86_64-linux-gnu/libglib-2.0.so.0.4400.1)
==10048==    by 0x5388B60: g_string_append_vprintf (in /lib/x86_64-linux-gnu/libglib-2.0.so.0.4400.1)
==10048==    by 0x5388D83: g_string_append_printf (in /lib/x86_64-linux-gnu/libglib-2.0.so.0.4400.1)
==10048==    by 0x539D034: g_variant_print_string (in /lib/x86_64-linux-gnu/libglib-2.0.so.0.4400.1)
==10048==    by 0x539C92C: g_variant_print (in /lib/x86_64-linux-gnu/libglib-2.0.so.0.4400.1)
==10048==    by 0x4E5F713: log_key (hwdriver.c:597)
==10048==    by 0x4E5FCD5: sr_config_set (hwdriver.c:752)
==10048==    by 0x408C1A: run_session (session.c:661)
==10048==    by 0x404FC5: main (main.c:267)
==10048==
==10048== 16 bytes in 1 blocks are definitely lost in loss record 18 of 37
==10048==    at 0x4C2DEAE: realloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==10048==    by 0x536C85D: g_realloc (in /lib/x86_64-linux-gnu/libglib-2.0.so.0.4400.1)
==10048==    by 0x53877C6: ??? (in /lib/x86_64-linux-gnu/libglib-2.0.so.0.4400.1)
==10048==    by 0x5388B60: g_string_append_vprintf (in /lib/x86_64-linux-gnu/libglib-2.0.so.0.4400.1)
==10048==    by 0x5388D83: g_string_append_printf (in /lib/x86_64-linux-gnu/libglib-2.0.so.0.4400.1)
==10048==    by 0x539D034: g_variant_print_string (in /lib/x86_64-linux-gnu/libglib-2.0.so.0.4400.1)
==10048==    by 0x539C92C: g_variant_print (in /lib/x86_64-linux-gnu/libglib-2.0.so.0.4400.1)
==10048==    by 0x4E5F713: log_key (hwdriver.c:597)
==10048==    by 0x4E5FBDD: sr_config_get (hwdriver.c:709)
==10048==    by 0x4080D7: datafeed_in (session.c:196)
==10048==    by 0x4E5D47E: sr_session_send (session.c:1192)
==10048==    by 0x4E62682: std_session_send_df_header (std.c:101)