libxl: Resolve Coverity FORWARD_NULL

Since the strtok_r call in libxlCapsInitGuests expects a non NULL first
parameter when the third parameter is NULL, we need to check that
the returned 'capabilities' from a libxl_get_version_info call is
not NULL and error out if so since the code expects it.
Signed-off-by: NJohn Ferlan <jferlan@redhat.com>

src/libxl/libxl_conf.c

@@ -319,6 +319,12 @@ libxlCapsInitGuests(libxl_ctx *ctx, virCapsPtr caps)
         return -1;
     }
 
+    if (!ver_info->capabilities) {
+        virReportError(VIR_ERR_INTERNAL_ERROR, "%s",
+                       _("Failed to get capabilities from libxenlight"));
+        return -1;
+    }
+
     err = regcomp(&regex, XEN_CAP_REGEX, REG_EXTENDED);
     if (err != 0) {
         char error[100];