lasmerge: address of xyz_scale always evaluates as true

[mloskot]

While building libLAS with GCC 4.2.3, two suspicious warnings are thrown:

lasmerge.c:343: warning: the address of ‘xyz_scale’ will always evaluate as ‘true’
lasmerge.c:351: warning: the address of ‘xyz_offset’ will always evaluate as ‘true’

This is because these arrays are allocated on stack, so testing their addresses makes little sense.

Probably, instead of testing arrays against null, some dedicated boolean flags should be used, indicating that offset or scale values were given.