patch 8.1.0751: some regexp errors are not tested

Problem:    Some regexp errors are not tested.
Solution:   Add a test function.
diff --git a/src/version.c b/src/version.c
index b0f4886..f84da1d 100644
--- a/src/version.c
+++ b/src/version.c
@@ -796,6 +796,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    751,
+/**/
     750,
 /**/
     749,