[client] Fix compiler warnings about potentially uninitialized variables

Build failed with _FORTIFY_SOURCE enabled because the compiler couldn't
ensure the switch statements didn't hit the default arm and thus wouldn't
define the variables. Adding a statically failing assert makes sure that
all code paths either define the variables or fail early.

$ cd client
$ env CFLAGS='-O1 -D_FORTIFY_SOURCE=1' cmake -B build/
$ make -C build
[...]
client/renderers/EGL/egl.c: In function ‘egl_calc_mouse_size’:
client/renderers/EGL/egl.c:299:36: error: ‘h’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
  299 |         (this->mouseHeight * (1.0f / h)) * this->scaleY
      |                              ~~~~~~^~~~

client/src/main.c

@@ -192,6 +192,9 @@ static void updatePositionInfo(void)
       srcW = g_state.srcSize.y;
       srcH = g_state.srcSize.x;
       break;
+
+    default:
+      assert(!"unreachable");
   }
 
   if (params.keepAspect)
@@ -1242,6 +1245,9 @@ inline static void localCurToGuest(struct DoublePoint *guest)
         * g_cursor.scale.y;
       guest->y = (point.x - g_state.dstRect.x) * g_cursor.scale.x;
       break;
+
+    default:
+      assert(!"unreachable");
   }
 }