xen: Resolve Coverity NEGATIVE_RETURNS

Coverity notes that if the call to virBitmapParse() returns a negative
value, then when we jump to the error label, the call to
virCapabilitiesClearHostNUMACellCPUTopology() will have issues
with the negative nb_cpus
Signed-off-by: NJohn Ferlan <jferlan@redhat.com>

src/xen/xend_internal.c

@@ -1112,7 +1112,8 @@ sexpr_to_xend_topology(const struct sexpr *root, virCapsPtr caps)
  parse_error:
     virReportError(VIR_ERR_XEN_CALL, "%s", _("topology syntax error"));
  error:
-    virCapabilitiesClearHostNUMACellCPUTopology(cpuInfo, nb_cpus);
+    if (nb_cpus > 0)
+        virCapabilitiesClearHostNUMACellCPUTopology(cpuInfo, nb_cpus);
     VIR_FREE(cpuInfo);
     return -1;
 }