patch 8.2.4829: a key may be simplified to NUL

Problem:    A key may be simplified to NUL.
Solution:   Use K_ZERO instead.  Use macros instead of hard coded values.
            (closes #10290)
diff --git a/src/getchar.c b/src/getchar.c
index 73cf420..4caa89e 100644
--- a/src/getchar.c
+++ b/src/getchar.c
@@ -1637,7 +1637,11 @@
     if (*modifiers & MOD_MASK_CTRL)
     {
 	if ((c >= '`' && c <= 0x7f) || (c >= '@' && c <= '_'))
+	{
 	    c &= 0x1f;
+	    if (c == NUL)
+		c = K_ZERO;
+	}
 	else if (c == '6')
 	    // CTRL-6 is equivalent to CTRL-^
 	    c = 0x1e;
@@ -3661,7 +3665,7 @@
 	    for (;;)
 	    {
 		len = ui_inchar(dum, DUM_LEN, 0L, 0);
-		if (len == 0 || (len == 1 && dum[0] == 3))
+		if (len == 0 || (len == 1 && dum[0] == Ctrl_C))
 		    break;
 	    }
 	    return retesc;