Avoid a potential underflow when iterating over invalid inputs

The forward iteration logic already bounds-check for m_it != m_string->end(),
do the same for the backward iteration. The issue with the assert is
that the assert() might not be compiled in, and it is happening after
the dereference, so it was too late.

1 files changed, 19 insertions, 0 deletions

diff --git a/tests/SelfTest/IntrospectiveTests/TextFlow.tests.cpp b/tests/SelfTest/IntrospectiveTests/TextFlow.tests.cpp
index fc344c0d..5f62f18d 100644
--- a/tests/SelfTest/IntrospectiveTests/TextFlow.tests.cpp
+++ b/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][approvals]" ) {
+    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