Fix #27096 - Do not sort a wasm section vector that failed to parse or is empty ##bin - #27118

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Description

parse_vec returns NULL when a section has no entries , and parse_sub_section_vec then sorted that NULL vector , so the reproducer from #27096 , an export section with count 0 , aborted with assertions on and printed a warning without them . the sort is skipped when there is nothing to sort .

the new r2r test writes the same 11 bytes into malloc:// and loads them with oba , so no binary fixture is needed . it fails before the change , the assertion aborts , and passes after . db/formats/web_assembly 10 OK , db/anal/wasm 22 OK , db/cmd/cmd_iw 21 OK . rabin2 -I and rabin2 -e on the reproducer exit 0 with R2_DEBUG_ASSERT=1 .