hmp: fix "dump-quest-memory" segfault (arm)

Running QEMU with
    qemu-system-aarch64 -M none -nographic -m 256
and executing
    dump-guest-memory /dev/null 0 8192
results in segfault

Fix by checking if we have CPU, and exit with
error if there is no CPU:

    (qemu) dump-guest-memory /dev/null
    this feature or command is not currently supported

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Message-Id: <20170913142036.2469-3-lvivier@redhat.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Tested-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

target/arm/arch_dump.c

@@ -273,11 +273,18 @@ int arm_cpu_write_elf32_note(WriteCoreDumpFunction f, CPUState *cs,
 int cpu_get_dump_info(ArchDumpInfo *info,
                       const GuestPhysBlockList *guest_phys_blocks)
 {
-    ARMCPU *cpu = ARM_CPU(first_cpu);
-    CPUARMState *env = &cpu->env;
+    ARMCPU *cpu;
+    CPUARMState *env;
     GuestPhysBlock *block;
     hwaddr lowest_addr = ULLONG_MAX;
 
+    if (first_cpu == NULL) {
+        return -1;
+    }
+
+    cpu = ARM_CPU(first_cpu);
+    env = &cpu->env;
+
     /* Take a best guess at the phys_base. If we get it wrong then crash
      * will need '--machdep phys_offset=<phys-offset>' added to its command
      * line, which isn't any worse than assuming we can use zero, but being