dbghelp: Fix PDB processing using the FPO stream instead of FPOEXT
Since wine 9.3 (particularly this commit), pdb_process_internal mistakenly sets fpoext_stream to the FPO stream (sub_streams[PDB_SIDX_FPO]) instead of the FPOEXT stream (sub_streams[PDB_SIDX_FPOEXT]), causing the PDB file to be improperly processed.
This merge request fixes that oversight.