Ensure ConvexPolygonShape support count variable is initialised

(cherry picked from commit 0d4d8925b7ebaf608e0b3c2abf0d8213d9ce95f7)

servers/physics_2d/shape_2d_sw.cpp

@@ -543,6 +543,7 @@ void ConvexPolygonShape2DSW::get_supports(const Vector2 &p_normal, Vector2 *r_su
 
 	int support_idx = -1;
 	real_t d = -1e10;
+	r_amount = 0;
 
 	for (int i = 0; i < point_count; i++) {
 
@@ -563,7 +564,7 @@ void ConvexPolygonShape2DSW::get_supports(const Vector2 &p_normal, Vector2 *r_su
 		}
 	}
 
-	ERR_FAIL_COND(support_idx == -1);
+	ERR_FAIL_COND_MSG(support_idx == -1, "Convex polygon shape support not found.");
 
 	r_amount = 1;
 	r_supports[0] = points[support_idx].pos;