patch 8.1.1061: when substitute string throws error, substitute happens anyway

Problem:    When substitute string throws error, substitute happens anyway.
Solution:   Skip substitution when aborting. (closes #4161)
diff --git a/src/ex_cmds.c b/src/ex_cmds.c
index 1b73ba3..2544124 100644
--- a/src/ex_cmds.c
+++ b/src/ex_cmds.c
@@ -5574,7 +5574,12 @@
 				    sub_firstlnum - regmatch.startpos[0].lnum,
 				    sub, sub_firstline, FALSE, p_magic, TRUE);
 #ifdef FEAT_EVAL
-		/* Don't keep flags set by a recursive call. */
+		// If getting the substitute string caused an error, don't do
+		// the replacement.
+		if (aborting())
+		    goto skip;
+
+		// Don't keep flags set by a recursive call.
 		subflags = subflags_save;
 		if (subflags.do_count)
 		{