1.0.47.1: fix longstanding bug in os_get_runtime_executable_path on darwin

1.0.47.1: fix longstanding bug in os_get_runtime_executable_path on darwin

 * in os_get_runtime_executable_path, we were wrongly assuming that
   size would be set to the actual size of the string on a successful
   call to NSGetExecutablePath. This isn't the case and we were
   setting path[1025] = '\0'; causing things to break with XCode 4's
   CLANG compiler.

 * While we're at it, we should check to make that runtime_path is not
   NULL before we free it, as there is a code path where we might end
   up trying to free runtime_path when it was NULL but
   saved_runtime_path was non-NULL.