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, 2 insertions, 2 deletions

diff --git a/src/catch2/internal/catch_textflow.cpp b/src/catch2/internal/catch_textflow.cpp
index 3979048f..0646b10f 100644
--- a/src/catch2/internal/catch_textflow.cpp
+++ b/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--;
             }
         }