From commit e19afd56, we know target-arm restricts the list of
queryable cpu models to 'max', 'host', and the current type when
KVM is in use. The logic works well until using machine type none.

For machine type none, cpu_type will be null, and strlen(cpu_type)
will terminate process. So I add a check above it.

This won't affect i386 and s390x, because they are not invovled
current_cpu.

Signed-off-by: Liang Yan <lyan@suse.com>
---
 v2: fix code style issue
---
 target/arm/monitor.c | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/target/arm/monitor.c b/target/arm/monitor.c
index 9725dfff16..3350cd65d0 100644
--- a/target/arm/monitor.c
+++ b/target/arm/monitor.c
@@ -137,17 +137,19 @@ CpuModelExpansionInfo *qmp_query_cpu_model_expansion(CpuModelExpansionType type,
     }
 
     if (kvm_enabled()) {
-        const char *cpu_type = current_machine->cpu_type;
-        int len = strlen(cpu_type) - strlen(ARM_CPU_TYPE_SUFFIX);
         bool supported = false;
 
         if (!strcmp(model->name, "host") || !strcmp(model->name, "max")) {
             /* These are kvmarm's recommended cpu types */
             supported = true;
-        } else if (strlen(model->name) == len &&
-                   !strncmp(model->name, cpu_type, len)) {
-            /* KVM is enabled and we're using this type, so it works. */
-            supported = true;
+        } else if (current_machine->cpu_type) {
+            const char *cpu_type = current_machine->cpu_type;
+            int len = strlen(cpu_type) - strlen(ARM_CPU_TYPE_SUFFIX);
+            if (strlen(model->name) == len &&
+                    !strncmp(model->name, cpu_type, len)) {
+                /* KVM is enabled and we're using this type, so it works. */
+                supported = true;
+            }
         }
         if (!supported) {
             error_setg(errp, "We cannot guarantee the CPU type '%s' works "
-- 
2.25.0