patch 8.1.1509: cmdline_row can become negative, causing a crash

Problem:    Cmdline_row can become negative, causing a crash.
Solution:   Make sure cmdline_row does not become negagive. (closes #4102)
diff --git a/src/misc1.c b/src/misc1.c
index 7ef7e78..f5549fd 100644
--- a/src/misc1.c
+++ b/src/misc1.c
@@ -1447,9 +1447,9 @@
     i = get_number(TRUE, mouse_used);
     if (KeyTyped)
     {
-	/* don't call wait_return() now */
-	/* msg_putchar('\n'); */
-	cmdline_row = msg_row - 1;
+	// don't call wait_return() now
+	if (msg_row > 0)
+	    cmdline_row = msg_row - 1;
 	need_wait_return = FALSE;
 	msg_didany = FALSE;
 	msg_didout = FALSE;