rts: print incorrect prev_what_next

Moritz Angermann reports mysterious rts crash:
  A: link: internal error: schedule: invalid what_next field
  A:     (GHC version 8.3.20170321 for arm_none_linux_android)

This change prints actual prev_what_next value.

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>

1 files changed, 1 insertions, 1 deletions

diff --git a/rts/Schedule.c b/rts/Schedule.c
index a4d0c1da80..b77e7a21ef 100644
--- a/rts/Schedule.c
+++ b/rts/Schedule.c
@@ -473,7 +473,7 @@ run_thread:
         break;
 
     default:
-        barf("schedule: invalid what_next field");
+        barf("schedule: invalid prev_what_next=%u field", prev_what_next);
     }
 
     cap->in_haskell = false;