Get maxima_pid from wxExecute().

Until now, the pid was not set (and -1
was printed during the logging). This
(hopefully) helps with issues, where
the "not killing" of Maxima processes
was reported, e.g. #1199, #1824, #1922
or #1963.

src/wxMaxima.cpp

@@ -2691,9 +2691,8 @@ bool wxMaxima::StartMaxima(bool force) {
       environment["MAXIMA_AUTH_CODE"] = m_maximaAuthString;
 
       env->env = std::move(environment);
-      if (wxExecute(command, wxEXEC_ASYNC | wxEXEC_HIDE_CONSOLE | wxEXEC_MAKE_GROUP_LEADER,
-                    m_maximaProcess,
-                    env.get()) <= 0) {
+      m_pid=wxExecute(command, wxEXEC_ASYNC | wxEXEC_HIDE_CONSOLE | wxEXEC_MAKE_GROUP_LEADER, m_maximaProcess, env.get());
+      if (m_pid <= 0) {
         StatusMaximaBusy(StatusBar::MaximaStatus::process_wont_start);
         StatusText(_("Cannot start the maxima binary"));
         m_maximaProcess = NULL;