fix(tui): avoid memory leak and compiler warning on Windows (#34225)

Problem:  On Windows, the value of `term` is overwritten without freeing
          the old allocated value, which may lead to a memory leak.
	  GCC also gives a "incompatible pointer type" warning about
	  passing `*term` to os_tty_guess_term().
Solution: Don't override the old allocated value, and copy the guessed
          value to `term` if its old value is NULL.

src/nvim/os/env.c

@@ -147,9 +147,6 @@ bool os_env_exists(const char *name, bool nonempty)
 /// Sets an environment variable.
 ///
 /// Windows (Vim-compat): Empty string (:let $FOO="") undefines the env var.
-///
-/// @warning Existing pointers to the result of os_getenv("foo") are
-///          INVALID after os_setenv("foo", …).
 int os_setenv(const char *name, const char *value, int overwrite)
   FUNC_ATTR_NONNULL_ALL
 {

src/nvim/tui/tui.c

@@ -386,10 +386,12 @@ static void terminfo_start(TUIData *tui)
 
   char *term = os_getenv("TERM");
 #ifdef MSWIN
-  os_tty_guess_term(&term, tui->out_fd);
-  os_setenv("TERM", term, 1);
-  // Old os_getenv() pointer is invalid after os_setenv(), fetch it again.
-  term = os_getenv("TERM");
+  const char *guessed_term = NULL;
+  os_tty_guess_term(&guessed_term, tui->out_fd);
+  if (term == NULL && guessed_term != NULL) {
+    term = xstrdup(guessed_term);
+    os_setenv("TERM", guessed_term, 1);
+  }
 #endif
 
   // Set up unibilium/terminfo.