fix segfault on empty points

Author: mourner

include/kdbush.hpp

@@ -51,6 +51,8 @@ class KDBush {
         assert(points.empty());
         const TIndex size = static_cast<TIndex>(std::distance(points_begin, points_end));
 
+        if (size == 0) return;
+
         points.reserve(size);
         ids.reserve(size);
 

test.cpp

@@ -42,8 +42,14 @@ static void testRadius() {
     assert(std::equal(expectedIds.begin(), expectedIds.end(), result.begin()));
 }
 
+static void testEmpty() {
+    auto emptyPoints = std::vector<TPoint>{};
+    kdbush::KDBush<TPoint> index(emptyPoints);
+}
+
 int main() {
     testRange();
     testRadius();
+    testEmpty();
     return 0;
 }