Fix breakpoint trigger condition

The breakpoint trigger condition included a check that if prev_line was zero,
no line transition would ever be detected.  This means that if a breakpoint
is set on the first line of a function, it would never get triggered - not
even after execution had proceeded beyond it.

Change the condition so that prev_line can be zero, and a transition 0->1 is
generated.  This allows breakpoints at line 1 to match.

src/duk_js_executor.c

@@ -1400,7 +1400,7 @@ DUK_LOCAL void duk__interrupt_handle_debugger(duk_hthread *thr, duk_bool_t *out_
 					break;
 				}
 				DUK_ASSERT(bp->filename != NULL);
-				if (act->prev_line != 0 && act->prev_line < bp->line && line >= bp->line) {
+				if (act->prev_line < bp->line && line >= bp->line) {
 					DUK_D(DUK_DPRINT("BREAKPOINT TRIGGERED at %!O:%ld",
 					                 (duk_heaphdr *) bp->filename, (long) bp->line));