Fix error checks to popen in hxemem.c
Fix error checks to popen in hxemem.c

The popen man page says it will only return NULL on error.

This removes checking against -1.  These unnecessary checks were also
producing a lot warnings like:
   ../hxemem64/hxemem.c:1131:23: warning: comparison between pointer and integer

These warnings are now gone.

Signed-off-by: Michael Neuling <7b0ab45a730e48a69239010b5b8fe5fa4e8eaac6@neuling.org>

stewart-ibm/HTX,stewart-ibm/HTX,open-power/HTX,stewart-ibm/HTX,open-power/HTX,open-power/HTX,open-power/HTX,open-power/HTX,stewart-ibm/HTX,stewart-ibm/HTX