patch 8.2.3470: crash with error in :catch and also in :finally

Problem:    Crash with error in :catch and also in :finally.
Solution:   Only discard an exception if there is one. (closes #8954)
diff --git a/src/ex_eval.c b/src/ex_eval.c
index 259cf67..503a3ae 100644
--- a/src/ex_eval.c
+++ b/src/ex_eval.c
@@ -2374,7 +2374,8 @@
 		    default:
 			if (cstack->cs_flags[idx] & CSF_FINALLY)
 			{
-			    if (cstack->cs_pending[idx] & CSTP_THROW)
+			    if ((cstack->cs_pending[idx] & CSTP_THROW)
+				    && cstack->cs_exception[idx] != NULL)
 			    {
 				// Cancel the pending exception.  This is in the
 				// finally clause, so that the stack of the