patch 9.0.1848: [security] buffer-overflow in vim_regsub_both()

Problem:  buffer-overflow in vim_regsub_both()
Solution: Check remaining space

Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/testdir/crash/vim_regsub_both b/src/testdir/crash/vim_regsub_both
new file mode 100644
index 0000000..a82b205
--- /dev/null
+++ b/src/testdir/crash/vim_regsub_both
@@ -0,0 +1,10 @@
+fu R()
+sil!norm0z=
+endf
+cal R()
+s/\%')/\=R()
+d
+no0 normyynore sm:vs0@vvvvvvvvvvse()dir(¼Xtest=csd{so88
+vs
+0scr
+so