s390x/tcg: Reset exception_index to -1 instead of 0

We use the marker "-1" for "no exception". s390_cpu_do_interrupt() might
get confused by that.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: David Hildenbrand <david@redhat.com>

target/s390x/mem_helper.c

@@ -1747,7 +1747,7 @@ uint32_t HELPER(tprot)(CPUS390XState *env, uint64_t a1, uint64_t a2)
 
     if (env->int_pgm_code == PGM_PROTECTION) {
         /* retry if reading is possible */
-        cs->exception_index = 0;
+        cs->exception_index = -1;
         if (!s390_cpu_virt_mem_check_read(cpu, a1, 0, 1)) {
             /* Fetching permitted; storing not permitted */
             return 1;
@@ -1757,7 +1757,7 @@ uint32_t HELPER(tprot)(CPUS390XState *env, uint64_t a1, uint64_t a2)
     switch (env->int_pgm_code) {
     case PGM_PROTECTION:
         /* Fetching not permitted; storing not permitted */
-        cs->exception_index = 0;
+        cs->exception_index = -1;
         return 2;
     case PGM_ADDRESSING:
     case PGM_TRANS_SPEC:
@@ -1767,7 +1767,7 @@ uint32_t HELPER(tprot)(CPUS390XState *env, uint64_t a1, uint64_t a2)
     }
 
     /* Translation not available */
-    cs->exception_index = 0;
+    cs->exception_index = -1;
     return 3;
 }