libxl: avoid a dereference of a null pointer
libxl: avoid a dereference of a null pointer

Variable 'l_disk' initialized to a null pointer value, control jumps to 'case
VIR_DOMAIN_DISK_DEVICE_DISK and then taking false branch, Within the expansion
of the macro 'libxlError': Field access results in a dereference of a null
pointer (loaded from variable 'l_disk').

* src/libxl/libxl_driver.c: Field access results in a dereference of a null
  pointer (loaded from variable 'l_disk')

Signed-off-by: Alex Jia <e236faec8cb8371bfb245ba3769a84afc9a11438@redhat.com>

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