libxl: Fix Coverity warning

libxl: Fix Coverity warning

John Ferlan reported the following Coverity warning:

In libxlDomainCoreDump() Coverity has noted a FORWARD_NULL reference:

2004 	    if ((flags & VIR_DUMP_CRASH) && !vm->persistent) {
2005 	        virDomainObjListRemove(driver->domains, vm);

(20) Event assign_zero: 	Assigning: "vm" = "NULL".
Also see events: 	[var_deref_model]

2006 	        vm = NULL;
2007 	    }
2008
2009 	    ret = 0;
2010
2011 	cleanup_unpause:

(21) Event var_deref_model: 	Passing null pointer "vm" to function
     "virDomainObjIsActive(virDomainObjPtr)", which dereferences it. [details]
Also see events: 	[assign_zero]

2012 	    if (virDomainObjIsActive(vm) && paused) {
2013 	        if (libxl_domain_unpause(priv->ctx, dom->id) != 0) {
2014 	            virReportError(VIR_ERR_INTERNAL_ERROR,

Removing the vm from domain obj list and setting it to NULL can be
done in the previous 'if (flags & VIR_DUMP_CRASH)' conditional.  Fix
the Coverity warning by ensuring vm is not NULL before testing if it
is still active.