security: Remove unnecessary checks for mgr == NULL

Coverity found the DACGenLabel was checking for mgr == NULL after a
possible dereference; however, in order to get into the function the
virSecurityManagerGenLabel would have already dereferenced sec_managers[i]
so the check was unnecessary. Same check is made in SELinuxGenSecurityLabel.

src/security/security_dac.c

@@ -913,12 +913,6 @@ virSecurityDACGenLabel(virSecurityManagerPtr mgr,
     virSecurityLabelDefPtr seclabel;
     virSecurityDACDataPtr priv = virSecurityManagerGetPrivateData(mgr);
 
-    if (mgr == NULL) {
-        virReportError(VIR_ERR_INTERNAL_ERROR, "%s",
-                       _("invalid security driver"));
-        return rc;
-    }
-
     seclabel = virDomainDefGetSecurityLabelDef(def, SECURITY_DAC_NAME);
     if (seclabel == NULL) {
         return rc;

src/security/security_selinux.c

@@ -560,12 +560,6 @@ virSecuritySELinuxGenSecurityLabel(virSecurityManagerPtr mgr,
     virSecuritySELinuxDataPtr data;
     const char *baselabel;
 
-    if (mgr == NULL) {
-        virReportError(VIR_ERR_INTERNAL_ERROR,
-                       "%s", _("invalid security driver"));
-        return rc;
-    }
-
     seclabel = virDomainDefGetSecurityLabelDef(def, SECURITY_SELINUX_NAME);
     if (seclabel == NULL) {
         return rc;