Improve tests and fix error context handling

.swiftformat

@@ -6,3 +6,4 @@
 --operatorfunc nospace
 --ifdef noindent
 --stripunusedargs closure-only
+--disable andOperator

Sources/XMLCoder/Auxiliaries/XMLStackParser.swift

@@ -48,7 +48,8 @@ class XMLStackParser: NSObject {
             ))
         }
 
-        guard errorContextLength > 0 else {
+        // `lineNumber` isn't 0-indexed, so 0 is an invalid value for context
+        guard errorContextLength > 0 && xmlParser.lineNumber > 0 else {
             throw error
         }
 

Tests/XMLCoderTests/ErrorContextTest.swift

@@ -14,6 +14,29 @@ final class ErrorContextTest: XCTestCase {
         let value: [String: Int]
     }
 
+    func testErrorContextFirstLine() {
+        let decoder = XMLDecoder()
+        decoder.errorContextLength = 8
+
+        let xmlString = "<blah //>"
+        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 2:
+            `<blah `
+            """)
+        }
+    }
+
     func testErrorContext() {
         let decoder = XMLDecoder()
         decoder.errorContextLength = 8