Guard clkcnt division against a zero line count

get_status() divides before check_linecnt() validates totlines; with no
lines measured the divisor is zero (RISC-V returns all ones silently).

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WiXHy7Jhh5yTdsBaenucgw

software/sys_controller/av_controller.c

@@ -369,7 +369,7 @@ status_t get_status(tvp_sync_input_t syncinput)
     pcnt_field = (unsigned long)sc->fe_status2.pcnt_field;
     hsync_width = (unsigned long)sc->fe_status2.hsync_width;
 
-    clkcnt = pcnt_field/(totlines>>!progressive);
+    clkcnt = (totlines>>!progressive) ? pcnt_field/(totlines>>!progressive) : 0;
     valid_mode = (pcnt_field > 0) && check_linecnt(progressive, totlines);
 
     // Check sync activity