Fix coding.c subscript error

* coding.c (CODING_ISO_INVOKED_CHARSET):
Avoid undefined behavior if CODING_ISO_INVOCATION returns negative.

1 files changed, 2 insertions, 1 deletions

diff --git a/src/coding.c b/src/coding.c
index 77583d0d950..8dc39e74422 100644
--- a/src/coding.c
+++ b/src/coding.c
@@ -395,7 +395,8 @@ static Lisp_Object Vbig5_coding_system;
 #define CODING_ISO_BOL(coding)  \
   ((coding)->spec.iso_2022.bol)
 #define CODING_ISO_INVOKED_CHARSET(coding, plane)       \
-  CODING_ISO_DESIGNATION ((coding), CODING_ISO_INVOCATION ((coding), (plane)))
+  (CODING_ISO_INVOCATION (coding, plane) < 0 ? -1       \
+   : CODING_ISO_DESIGNATION (coding, CODING_ISO_INVOCATION (coding, plane)))
 #define CODING_ISO_CMP_STATUS(coding)   \
   (&(coding)->spec.iso_2022.cmp_status)
 #define CODING_ISO_EXTSEGMENT_LEN(coding)       \