Fix incorrect exception_index use

env->exception_index should be cleared with -1, not 0.

See also 821b19fe923ac49a24cdb4af902584fdd019cee6.

Spotted by Igor Kovalenko.

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
diff --git a/target-sh4/helper.c b/target-sh4/helper.c
index f38e6ab..d2ff669 100644
--- a/target-sh4/helper.c
+++ b/target-sh4/helper.c
@@ -38,7 +38,7 @@
 			     int mmu_idx, int is_softmmu)
 {
     env->tea = address;
-    env->exception_index = 0;
+    env->exception_index = -1;
     switch (rw) {
     case 0:
         env->exception_index = 0x0a0;