Use explicit boolean comparison in OOM check

Use explicit boolean comparison in OOM check

GCC 7 gets upset by

   if (!tmp && (size * count))

warning

  util/viralloc.c: In function 'virReallocN':
  util/viralloc.c:246:23: error: '*' in boolean context, suggest '&&' instead [-Werror=int-in-bool-context]
     if (!tmp && (size * count)) {
                 ~~~~~~^~~~~~~~

Keep it happy by adding != 0 to the right hand expression
so it realizes we really are wanting to treat the result
of the arithmetic expression as a boolean

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>