Avoid a potential underflow when iterating over invalid inputs

src/catch2/internal/catch_textflow.cpp

@@ -148,8 +148,8 @@ namespace Catch {
                 m_it--;
             }
             // Skip back over UTF-8 continuation bytes to the leading byte
-            while ( isUtf8ContinuationByte( *m_it ) ) {
-                assert( m_it != m_string->begin() );
+            while ( m_it != m_string->begin() &&
+                    isUtf8ContinuationByte( *m_it ) ) {
                 m_it--;
             }
         }

tests/SelfTest/IntrospectiveTests/TextFlow.tests.cpp

@@ -437,6 +437,25 @@ TEST_CASE( "TextFlow::AnsiSkippingString iterates UTF-8 codepoints",
     }
 }
 
+TEST_CASE( "TextFlow::AnsiSkippingString handles invalid UTF-8",
+           "[TextFlow][ansiskippingstring][regression]" ) {
+    SECTION( "Continuation byte at the start" ) {
+        // 0x80 is a continuation byte
+        AnsiSkippingString str( "\x80" );
+        auto it = str.end();
+        --it;
+        CHECK( it == str.begin() );
+        CHECK( *it == static_cast<char>( 0x80 ) );
+    }
+    SECTION( "Multiple continuation bytes at the start" ) {
+        AnsiSkippingString str( "\x80\x80\x80" );
+        auto it = str.end();
+        --it;
+        CHECK( it == str.begin() );
+        CHECK( *it == static_cast<char>( 0x80 ) );
+    }
+}
+
 TEST_CASE( "TextFlow::Column wraps UTF-8 text correctly",
            "[TextFlow][column][approvals]" ) {
     // "äöü äöü äöü" = 11 codepoints, 17 bytes