Bugfix: OPL parser stumbled on "%%" in text

In this case a zero character (`\0`) was inserted into the text, but
that's not an allowed UTF-8 character. The fix now inserts a `%`
instead.

Fixes #328.

Author: joto

include/osmium/io/detail/opl_parser_functions.hpp

@@ -144,6 +144,8 @@ namespace osmium {
              * Returns a pointer to next character that needs to be consumed.
              */
             inline void opl_parse_escaped(const char** data, std::string& result) {
+                assert(data);
+                assert(*data);
                 const char* s = *data;
                 uint32_t value = 0;
                 const int max_length = sizeof(value) * 2 /* hex chars per byte */;
@@ -154,7 +156,11 @@ namespace osmium {
                     }
                     if (*s == '%') {
                         ++s;
-                        append_codepoint_as_utf8(value, std::back_inserter(result));
+                        if (value == 0) {
+                            result += '%';
+                        } else {
+                            append_codepoint_as_utf8(value, std::back_inserter(result));
+                        }
                         *data = s;
                         return;
                     }
@@ -182,6 +188,8 @@ namespace osmium {
              * Returns a pointer to next character that needs to be consumed.
              */
             inline void opl_parse_string(const char** data, std::string& result) {
+                assert(data);
+                assert(*data);
                 const char* s = *data;
                 while (true) {
                     if (*s == '\0' || *s == ' ' || *s == '\t' || *s == ',' || *s == '=') {

include/osmium/io/detail/string_util.hpp

@@ -203,6 +203,7 @@ namespace osmium {
 
             inline void append_utf8_encoded_string(std::string& out, const char* data) {
                 static const char* lookup_hex = "0123456789abcdef";
+                assert(data);
                 const char* end_ptr = data + std::strlen(data);
 
                 while (data != end_ptr) {
@@ -236,6 +237,7 @@ namespace osmium {
             }
 
             inline void append_xml_encoded_string(std::string& out, const char* data) {
+                assert(data);
                 for (; *data != '\0'; ++data) {
                     switch (*data) {
                         case '&':  out += "&";  break;

test/t/io/test_opl_parser.cpp

@@ -104,7 +104,7 @@ TEST_CASE("Parse OPL: parse escaped") {
         const char* e = s + std::strlen(s);
         oid::opl_parse_escaped(&s, result);
         REQUIRE(result.size() == 1);
-        REQUIRE(result[0] == '\0');
+        REQUIRE(result[0] == '%');
         REQUIRE(s == e);
     }