libarchive: Avoid bogus conversion warning from PGI compiler

We cannot suppress PGI compiler warnings completely because even with
the "-w" flag the compiler still writes a message containing "compilation
completed with warnings" to stderr.

A warning is triggered by expressions like

  test ? NULL : ptr_to_const_char
  test ? ".." : ptr_to_const_char

that the PGI compiler handles incorrectly.  It chooses the pointer type
of the first option (either void* or char*) and warns about conversion
of the second without a cast.  Flip the expression logic to

  !test ? ptr_to_const_char : NULL
  !test ? ptr_to_const_char : ".."

to help the compiler choose the proper result type.

0 files changed, 0 insertions, 0 deletions