virsh: initialize str to NULL to solve a build issue

Fix a -Werror=maybe-uninitialized issue.
Signed-off-by: NChen Hanxiao <chenhanxiao@cn.fujitsu.com>
Signed-off-by: NJán Tomko <jtomko@redhat.com>

tools/virsh-domain-monitor.c

@@ -218,7 +218,7 @@ VIR_ENUM_IMPL(vshDomainPMSuspendedReason,
 static const char *
 vshDomainStateReasonToString(int state, int reason)
 {
-    const char *str;
+    const char *str = NULL;
     switch ((virDomainState) state) {
     case VIR_DOMAIN_NOSTATE:
         str = vshDomainNostateReasonTypeToString(reason);

tools/virsh-domain.c

@@ -10399,7 +10399,7 @@ VIR_ENUM_IMPL(vshDomainEventCrashed,
 static const char *
 vshDomainEventDetailToString(int event, int detail)
 {
-    const char *str;
+    const char *str = NULL;
     switch ((virDomainEventType) event) {
     case VIR_DOMAIN_EVENT_DEFINED:
         str = vshDomainEventDefinedTypeToString(detail);