patch 8.2.1680: Vim9: line number for compare error is wrong

Problem:    Vim9: line number for compare error is wrong.
Solution:   Set SOURCING_LNUM. (closes #6936)
diff --git a/src/vim9execute.c b/src/vim9execute.c
index e93817a..c2bd857 100644
--- a/src/vim9execute.c
+++ b/src/vim9execute.c
@@ -2126,6 +2126,7 @@
 		    exptype_T	exptype = iptr->isn_arg.op.op_type;
 		    int		ic = iptr->isn_arg.op.op_ic;
 
+		    SOURCING_LNUM = iptr->isn_lnum;
 		    typval_compare(tv1, tv2, exptype, ic);
 		    clear_tv(tv2);
 		    --ectx.ec_stack.ga_len;