microblaze: Dont segfault when singlestepping first insn.

Signed-off-by: Edgar E. Iglesias <edgar.iglesias@petalogix.com>

target-microblaze/op_helper.c

diff --git a/target-microblaze/op_helper.c b/target-microblaze/op_helper.c
index af89dcf..8a9cea2 100644
--- a/target-microblaze/op_helper.c
+++ b/target-microblaze/op_helper.c
@@ -251,6 +251,12 @@
                           int is_asi, int size)
 {
     CPUState *saved_env;
+
+    if (!cpu_single_env) {
+        /* XXX: ???   */
+        return;
+    }
+
     /* XXX: hack to restore env in all cases, even if not called from
        generated code */
     saved_env = env;