patch 8.2.3634: error for already defined function uses wrong line number

Problem:    Error for already defined function uses wrong line number.
Solution:   Set SOURCING_LNUM before giving the error message. (closes #9085)
diff --git a/src/userfunc.c b/src/userfunc.c
index 95d9c4a..602d3b7 100644
--- a/src/userfunc.c
+++ b/src/userfunc.c
@@ -4256,6 +4256,7 @@
 			&& (fp->uf_script_ctx.sc_sid != current_sctx.sc_sid
 			  || fp->uf_script_ctx.sc_seq == current_sctx.sc_seq)))
 	    {
+		SOURCING_LNUM = sourcing_lnum_top;
 		if (vim9script)
 		    emsg_funcname(e_name_already_defined_str, name);
 		else