Add test to case when error context size goes outside content size

Sources/XMLCoder/Auxiliaries/XMLStackParser.swift

@@ -51,8 +51,19 @@ class _XMLStackParser: NSObject {
             errorPosition += lines[i].count
         }
         errorPosition += xmlParser.columnNumber
-        let lowerBound = String.Index(encodedOffset: errorPosition - offset)
-        let upperBound = String.Index(encodedOffset: errorPosition + offset)
+
+        var lowerBoundIndex = 0
+        if errorPosition - offset > 0 {
+            lowerBoundIndex = errorPosition - offset
+        }
+
+        var upperBoundIndex = string.count
+        if errorPosition + offset < string.count {
+            upperBoundIndex = errorPosition + offset
+        }
+
+        let lowerBound = String.Index(encodedOffset: lowerBoundIndex)
+        let upperBound = String.Index(encodedOffset: upperBoundIndex)
 
         let context = string[lowerBound..<upperBound]
 

Tests/XMLCoderTests/ErrorContextTest.swift

@@ -46,6 +46,37 @@ final class ErrorContextTest: XCTestCase {
         }
     }
 
+    func testErrorContextSizeOutsizeContent() {
+        let decoder = XMLDecoder()
+        decoder.errorContextLength = 10
+
+        let xmlString =
+            """
+            container>
+                test1
+            </blah>
+            <container>
+                test2
+            </container>
+            """
+        let xmlData = xmlString.data(using: .utf8)!
+
+        XCTAssertThrowsError(try decoder.decode(Container.self,
+                                                from: xmlData)) { error in
+            guard case let DecodingError.dataCorrupted(ctx) = error,
+                let underlying = ctx.underlyingError else {
+                XCTAssert(false, "wrong error type thrown")
+                return
+            }
+
+            XCTAssertEqual(ctx.debugDescription, """
+            \(underlying.localizedDescription) \
+            at line 1, column 1:
+            `contai`
+            """)
+        }
+    }
+
     static var allTests = [
         ("testErrorContext", testErrorContext),
     ]

Tests/XMLCoderTests/ErrorContextTests.swift

@@ -0,0 +1,8 @@
+//
+//  ErrorContextTests.swift
+//  XMLCoder
+//
+//  Created by Matvii Hodovaniuk on 1/7/19.
+//
+
+import Foundation