Switch to fallback earlier if the input is not valid UTF-8 prefix

src/common/strconv.cpp

@@ -1446,6 +1446,26 @@ size_t wxMBConvUTF8::FromWChar(char *buf, size_t n,
     return len;
 }
 
+// checks if the input can be the beginning of a valid UTF-8 string
+bool wxIsUTF8Prefix(const char *src, size_t len)
+{
+    unsigned char l;
+    for ( size_t i = 0; i < len; ++i )
+    {
+        l = tableUtf8Lengths[(unsigned char)src[i]];
+        if ( !l )
+            return false; // invalid leading byte
+        while ( --l )
+        {
+            if ( ++i == len )
+                return true; // truncated sequence
+            if ( (src[i] & 0xC0) != 0x80 )
+                return false; // invalid continuation byte
+        }
+    }
+    return true;
+}
+
 // ============================================================================
 // UTF-16
 // ============================================================================