Problem: parse_cmdline() sets eap->cmdlinep to address of local parameter,
causing invalid memory access when expand_filename() tries to modify it.
This leads to crashes when typing '%' in user commands with preview=true
and complete=file.
Solution: Change parse_cmdline() signature to accept char **cmdline,
allowing cmdlinep to point to caller's variable for safe reallocation.
c7c3f9fc9c