patch 9.1.0022: Coverity complains about improper use of negative value

Problem:  Coverity complains about improper use of negative value
Solution: Add a condition to validate that keytyped is larger or equal
          to 0

Apparently patch 9.1.0006 made it more explicit for Coverity, that the
TOLOWER_LOC() macros do not handle negative values properly. However,
that condition has always been there even before that, so add a
condition to verify that keytyped is indeed at least 0

closes: #13824

Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/cindent.c b/src/cindent.c
index 6289422..cf18441 100644
--- a/src/cindent.c
+++ b/src/cindent.c
@@ -4108,7 +4108,8 @@
 		}
 		else
 		    // TODO: multi-byte
-		    if (keytyped == (int)p[-1] || (icase && keytyped < 256
+		    if (keytyped == (int)p[-1] || (icase
+				&& keytyped < 256 && keytyped >= 0
 			 && TOLOWER_LOC(keytyped) == TOLOWER_LOC((int)p[-1])))
 		{
 		    line = ml_get_cursor();