[mucurses] Attempt to fix keypress processing logic

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/hci/mucurses/kb.c

 			_wcursback( win );
 			wdelch( win );
 		} else {
-			if ( c >= KEY_MIN ) {
+			if ( c >= 32 && c <= 126 ) {
+				*(_str++) = c; n--;
+			} else {
 				switch(c) {
 				case KEY_LEFT :
 				case KEY_BACKSPACE :
 					break;
 				}
 			}
-			if ( c >= 32 && c <= 126 ) {
-				*(_str++) = c; n--;
-			}
 		}
 	}