Double free in `value_set get` - unsuccessful value_set read

[tatry]

1. Create any value_set in the parser.
2. Try to read its whole content. Result:

   {
     "IngressParserImpl_p_pvs": []
   double free or corruption (top)
   Aborted

   Exit code: 134

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Stack trace leading to free_btf (line numbers after applying PR #68):

free_btf                         btf.c:274
psabpf_table_entry_ctx_free      psabpf_table.c:57
psabpf_value_set_get_next_entry  psabpf_value_set.c:140
get_and_print_value_set_json     value_set.c:70
do_value_set_get                 value_set.c:194
cmd_select                       main.c:48
do_value_set                     main.c:158
cmd_select                       main.c:48
main                             main.c:188
__libc_start_main                0x00007ffff7d17083
_start                           0x000055555555b08e

Second stack trace leading to free_btf:

free_btf                       btf.c:274
psabpf_value_set_context_free  psabpf_value_set.c:51
do_value_set_get               value_set.c:198
cmd_select                     main.c:48
do_value_set                   main.c:158
cmd_select                     main.c:48
main                           main.c:188
__libc_start_main              0x00007ffff7d17083
_start                         0x000055555555b08e

Difference between these stack traces is that free_btf is called twice - once inside psabpf_value_set_get_next_entry and second time in psabpf_value_set_context_free. free_btf should be called only once in the later function.

There is second problem in psabpf_value_set_get_next_entry - each entry returned by this function must be freed, otherwise there is a memory leak.

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PVS definition (instance must be used later in the parser code):

value_set<bit<16>>(4) pvs;

Command to read PVS:

psabpf-ctl value-set get pipe 1 IngressParserImpl_p_pvs