AdssrEnvelope: Fix division-by-zero failure when going into decay (#5,#9

)

If the default values (ADSR mode on, Sustain level zero) are first
loaded, and then a patch setting ADSR mode, depending on which
order the parameters are set, this can result in a
division-by-zero in calculate_sustain_asymptote().

Fix this by recalculating sustain_asymptote before every
usage.

(The envelope code should really be rewritten to avoid
recalculating coef at every phase transition; coefs should
be set when the parameters are set, separately for
each envelope stage).

plugins/mimid/Engine/AdssrEnvelope.h

@@ -146,8 +146,8 @@ class AdssrEnvelope
 	void setSustain(float sus)
 	{
 		sustain = sus;
-		sustain_asymptote = calc_sustain_asymptote();
 		if (state == DEC || state == SUS) {
+			sustain_asymptote = calc_sustain_asymptote();
 			// Chase sustain level at decay rate, if sustain
 			// level changed in ADSR mode
 			if (Value > sustain) {
@@ -205,6 +205,7 @@ class AdssrEnvelope
 			if (Value > 1.0f) {
 				Value = 1.0f;
 				state = DEC;
+				sustain_asymptote = calc_sustain_asymptote();
 				coef = coef_dec(decay);
 				dir = 1;
 			}