patch 9.1.0399: block_editing errors out when using del

Problem:  block_editing errors out when using del
          (@Jamarley)
Solution: Change ins_len from size_t to int and
          properly check that it doesn't become negative

There is a check in os.c that verifies that `ins_len` does not become
negative:
```
if (pre_textlen >= 0 && (ins_len = len - pre_textlen - offset) > 0)
```
However this only works, if ins_len can actually become negative and
unfortunately, ins_len has been declared as `size_t` so instead of
becoming negative it will wrap around and be very large.

So let's define it as integer, after which the condition above
properly catches this condition.

fixes: #14734
closes: #14735

Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/ops.c b/src/ops.c
index e3e7827..cdc3080 100644
--- a/src/ops.c
+++ b/src/ops.c
@@ -1608,7 +1608,7 @@
 
     if (oap->block_mode)
     {
-	size_t			ins_len;
+	int			ins_len;
 	char_u			*firstline, *ins_text;
 	struct block_def	bd2;
 	int			did_indent = FALSE;