[Swift] Fix padding function overflow when bufSize is 0 (#5721)

* [Swift] Fix padding function overflow when bufSize is 0

[Swift] Generate linuxmain

* [Swift] Using the overflow addition operator to resolve integer overflow

swift/Sources/FlatBuffers/FlatBufferBuilder.swift

@@ -219,7 +219,7 @@ extension FlatBufferBuilder {
     ///   - bufSize: Current size of the buffer + the offset of the object to be written
     ///   - elementSize: Element size
     fileprivate func padding(bufSize: UInt32, elementSize: UInt32) -> UInt32 {
-        ((~bufSize) + 1) & (elementSize - 1)
+        ((~bufSize) &+ 1) & (elementSize - 1)
     }
     
     /// Prealigns the buffer before writting a new object into the buffer

tests/FlatBuffers.Test.Swift/Tests/FlatBuffers.Test.SwiftTests/FlatBuffersVectorsTests.swift

@@ -25,6 +25,14 @@ final class FlatBuffersVectors: XCTestCase {
         XCTAssertEqual(b.sizedByteArray, [4, 0, 0, 0, 5, 0, 0, 0, 1, 0, 0, 0, 2, 0, 0, 0, 3, 0, 0, 0, 4, 0, 0, 0, 5, 0, 0, 0])
     }
     
+    func testCreateEmptyIntArray() {
+        let numbers: [Int32] = []
+        let b = FlatBufferBuilder(initialSize: 20)
+        let o = b.createVector(numbers, size: numbers.count)
+        b.finish(offset: o)
+        XCTAssertEqual(b.sizedByteArray, [4, 0, 0, 0, 0, 0, 0, 0])
+    }
+
     func testCreateVectorOfStrings() {
         let strs = ["Denmark", "Norway"]
         let b = FlatBufferBuilder(initialSize: 20)

tests/FlatBuffers.Test.Swift/Tests/FlatBuffers.Test.SwiftTests/XCTestManifests.swift

@@ -72,6 +72,7 @@ extension FlatBuffersVectors {
     //   `swift test --generate-linuxmain`
     // to regenerate.
     static let __allTests__FlatBuffersVectors = [
+        ("testCreateEmptyIntArray", testCreateEmptyIntArray),
         ("testCreateIntArray", testCreateIntArray),
         ("testCreateSharedStringVector", testCreateSharedStringVector),
         ("testCreateVectorOfStrings", testCreateVectorOfStrings),