patch 8.1.1296: crash when using invalid command line argument

Problem:    Crash when using invalid command line argument.
Solution:   Check for options not being initialized.
diff --git a/src/term.c b/src/term.c
index bcc3dd3..462fe6f 100644
--- a/src/term.c
+++ b/src/term.c
@@ -3014,13 +3014,13 @@
     void
 term_push_title(int which)
 {
-    if ((which & SAVE_RESTORE_TITLE) && *T_CST != NUL)
+    if ((which & SAVE_RESTORE_TITLE) && T_CST != NULL && *T_CST != NUL)
     {
 	OUT_STR(T_CST);
 	out_flush();
     }
 
-    if ((which & SAVE_RESTORE_ICON) && *T_SSI != NUL)
+    if ((which & SAVE_RESTORE_ICON) && T_SSI != NULL && *T_SSI != NUL)
     {
 	OUT_STR(T_SSI);
 	out_flush();
@@ -3033,13 +3033,13 @@
     void
 term_pop_title(int which)
 {
-    if ((which & SAVE_RESTORE_TITLE) && *T_CRT != NUL)
+    if ((which & SAVE_RESTORE_TITLE) && T_CRT != NULL && *T_CRT != NUL)
     {
 	OUT_STR(T_CRT);
 	out_flush();
     }
 
-    if ((which & SAVE_RESTORE_ICON) && *T_SRI != NUL)
+    if ((which & SAVE_RESTORE_ICON) && T_SRI != NULL && *T_SRI != NUL)
     {
 	OUT_STR(T_SRI);
 	out_flush();