If a frequency of less than 17 (The lowest audible frequency for elephants...

Humans generally gan't hear anything below 20Hz) force to 17.  Fixes a one-
in-800 chance of segfault from sbbs_t::inkey() when it beeps.

src/xpdev/xpbeep.c

@@ -195,6 +195,8 @@ BOOL DLLCALL xptone(double freq, DWORD duration, enum WAVE_SHAPE shape)
 	int	i;
 	unsigned char	wave[S_RATE*15/2+1];
 
+	if(freq<17)
+		freq=17;
 	samples=S_RATE*duration/1000;
 	if(samples<=S_RATE/freq*2)
 		samples=S_RATE/freq*2;