[Nayana-Naik73]

In parse-only mode (bParseOnly, reached through DecodeParser) no reconstruction picture is allocated, so pDec->pData[0..2] are NULL. The four I_PCM macroblock handlers still computed the destination pointers up front:

codec/decoder/core/src/decode_slice.cpp:1830:50: runtime error: applying non-zero offset 16 to null pointer
    #0 WelsActualDecodeMbCavlcISlice(...) decode_slice.cpp:1830
    #1 WelsDecodeSlice(...) decode_slice.cpp
    #2 DecodeCurrentAccessUnit(...) decoder_core.cpp
    #3 WelsDecodeBs decoder.cpp
    #4 CWelsDecoder::DecodeParser(...) welsDecoderExt.cpp

pData[n] + offset on a null base is undefined behavior even though the pointers are only dereferenced inside the following if (!pCtx->pParam->bParseOnly) block. The same pattern exists in all four I_PCM paths: ParseIPCMInfoCabac (CABAC) and WelsActualDecodeMbCavlc{I,P,B}Slice (CAVLC). Moving the pointer computation inside that guard removes the UB without changing reconstruction behavior.

Reproduced with a crafted stream and with the conformance stream res/CVPCMNL1_SVA_C.264 (CAVLC I_PCM) decoded via the parse-only API under -fsanitize=undefined. Added DecoderParseSyntaxTest::TestParseOnlyIPcm, which parses CVPCMNL1_SVA_C.264 and test_cif_I_CABAC_PCM.264 in parse-only mode and covers all four sites; it trips UBSan before this change and is clean after.