patch 9.0.2114: overflow detection not accurate when adding digits

Problem:  overflow detection not accurate when adding digits
Solution: Use a helper function

Use a helper function to better detect overflows before adding integer
digits to a long or an integer variable respectively. Signal the
overflow to the caller function.

closes: #13539

Signed-off-by: Christian Brabandt <cb@256bit.org>
Signed-off-by: Michael Henry <vim@drmikehenry.com>
Signed-off-by: Ernie Rael <errael@raelity.com>
diff --git a/src/normal.c b/src/normal.c
index 16b4b45..61a19c1 100644
--- a/src/normal.c
+++ b/src/normal.c
@@ -2563,12 +2563,11 @@
 	    n /= 10;
 	else if (VIM_ISDIGIT(nchar))
 	{
-	    if (n > LONG_MAX / 10)
+	    if (vim_append_digit_long(&n, nchar - '0') == FAIL)
 	    {
 		clearopbeep(cap->oap);
 		break;
 	    }
-	    n = n * 10 + (nchar - '0');
 	}
 	else if (nchar == CAR)
 	{