Fix problem with repeated access to zip entry's data

Sources/ZipEntry.swift

@@ -85,6 +85,8 @@ public class ZipEntry: ContainerEntry {
                 localHeader!.lastModFileTime == cdEntry.lastModFileTime &&
                 localHeader!.lastModFileDate == cdEntry.lastModFileDate
                 else { throw ZipError.wrongLocalHeader }
+        } else {
+            pointerData.index += localHeader!.headerSize
         }
 
         let hasDataDescriptor = localHeader!.generalPurposeBitFlags & 0x08 != 0

Sources/ZipLocalHeader.swift

@@ -25,6 +25,8 @@ struct ZipLocalHeader {
     private(set) var accessTimestamp: UInt32?
     private(set) var creationTimestamp: UInt32?
 
+    let headerSize: Int
+
     init(_ pointerData: DataWithPointer) throws {
         // Check signature.
         guard pointerData.uint32() == 0x04034b50
@@ -47,6 +49,7 @@ struct ZipLocalHeader {
 
         let fileNameLength = pointerData.uint16().toInt()
         let extraFieldLength = pointerData.uint16().toInt()
+        self.headerSize = 30 + fileNameLength + extraFieldLength
 
         guard let fileName = ZipCommon.getStringField(pointerData, fileNameLength, useUtf8)
             else { throw ZipError.wrongTextField }

Tests/ZipTests.swift

@@ -151,6 +151,8 @@ class ZipTests: XCTestCase {
         }
 
         XCTAssertEqual(try? entries[0].data(), answerData)
+        // Test repeat of getting entry data (there was a problem with it).
+        XCTAssertEqual(try? entries[0].data(), answerData)
     }
 
     func testZipBZip2() {