tools/libx{l, c}: Fix trivial Coverity defects in migration v2 code

tools/libx{l, c}: Fix trivial Coverity defects in migration v2 code

All of these are UNUSED_VALUE defects where a default value is
unconditionally overwritten.  They are not particularly interesting,
bug wise, but keeping these defects at bay helps prevent real bugs
going unnoticed in the volume.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
CC: Ian Campbell <Ian.Campbell@citrix.com>
CC: Ian Jackson <Ian.Jackson@eu.citrix.com>
CC: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>