Bugfix: Check for readlink buffer overflow and handle gracefully

If readlink returns the size of the buffer, an overflow may have (safely) occurred.
Pass a buffer size of MAX_PATH+1 (the size of the actual buffer) to detect this scenario.

src/qt/guiutil.cpp

@@ -743,9 +743,10 @@ bool SetStartOnSystemStartup(bool fAutoStart)
     else
     {
         char pszExePath[MAX_PATH+1];
-        ssize_t r = readlink("/proc/self/exe", pszExePath, sizeof(pszExePath) - 1);
+        ssize_t r = readlink("/proc/self/exe", pszExePath, sizeof(pszExePath));
-        if (r == -1)
+        if (r == -1 || r > MAX_PATH) {
             return false;
+        }
         pszExePath[r] = '\0';
 
         fs::create_directories(GetAutostartDir());