Use a fixed-size buffer for log messages

libc++'s implementation of ostrstream seems to have incorrect
range-checking and does some out-of-bounds reads and writes whenever a
resize is needed, which results in crashes on 10.7 (but not 10.9 for
unknown reasons).

aegisub/libaegisub/common/log.cpp

@@ -84,7 +84,9 @@ decltype(LogSink::messages) LogSink::GetMessages() const {
 	return ret;
 }
 
-Message::Message(const char *section, Severity severity, const char *file, const char *func, int line) {
+Message::Message(const char *section, Severity severity, const char *file, const char *func, int line)
+: msg(buffer, sizeof buffer)
+{
 	sm.section = section;
 	sm.severity = severity;
 	sm.file = file;

aegisub/libaegisub/include/libaegisub/log.h

@@ -139,6 +139,7 @@ public:
 class Message {
 	std::ostrstream msg;
 	SinkMessage sm;
+	char buffer[2048];
 
 public:
 	Message(const char *section, Severity severity, const char *file, const char *func, int line);