Fix menu fizzle-out transition ending instantly (#747)

The decomp zeroes the slot-4 transition timer when starting the fizzle-out
(D_8018E7E0 = 0, the scalar that the gCurrentTransitionTime[4] out-of-bounds
index lands on). The rename in #309 turned that into
gCurrentTransitionTime[4] = arg0, which starts the timer at the full
duration, so the first frame sees it expired and completes the transition
immediately. Restores the dissolve when backing out of the menu and lets
the back sound play out.

src/menu_items.c

@@ -5750,7 +5750,7 @@ void func_8009E0F0(s32 arg0) {
         if (gTransitionDuration[4] >= 0x100U) {
             gTransitionDuration[4] = 0x000000FF;
         }
-        gCurrentTransitionTime[4] = arg0;
+        gCurrentTransitionTime[4] = 0;
         for (var_v0 = 0; var_v0 < 0x4B0; var_v0++) {
             sTKMK00_LowResBuffer[var_v0] = 0;
         }