Simplify tui_source_window_base::show_source_content

tui_source_window_base::show_source_content is not used outside the
class any more, so this makes it private.  Examining the callers shows
that it can't be called without source contents, so it can be
simplified as well.

gdb/ChangeLog
2019-09-20  Tom Tromey  <tom@tromey.com>

	* tui/tui-winsource.h (struct tui_source_window_base)
	<show_source_content>: Now private.
	* tui/tui-winsource.c
	(tui_source_window_base::show_source_content): Don't handle empty
	content case.

gdb/ChangeLog

@@ -1,3 +1,11 @@
+2019-09-20  Tom Tromey  <tom@tromey.com>
+
+	* tui/tui-winsource.h (struct tui_source_window_base)
+	<show_source_content>: Now private.
+	* tui/tui-winsource.c
+	(tui_source_window_base::show_source_content): Don't handle empty
+	content case.
+
 2019-09-20  Tom Tromey  <tom@tromey.com>
 
 	* tui/tui-layout.c (show_source_disasm_command)

gdb/tui/tui-winsource.c

@@ -238,15 +238,10 @@ tui_show_source_line (struct tui_source_window_base *win_info, int lineno)
 void
 tui_source_window_base::show_source_content ()
 {
-  if (!content.empty ())
-    {
-      int lineno;
+  gdb_assert (!content.empty ());
 
-      for (lineno = 1; lineno <= content.size (); lineno++)
-        tui_show_source_line (this, lineno);
-    }
-  else
-    erase_source_content ();
+  for (int lineno = 1; lineno <= content.size (); lineno++)
+    tui_show_source_line (this, lineno);
 
   check_and_display_highlight_if_needed ();
   refresh_window ();

gdb/tui/tui-winsource.h

@@ -76,6 +76,9 @@ struct tui_source_element
 
 struct tui_source_window_base : public tui_win_info
 {
+private:
+  void show_source_content ();
+
 protected:
   explicit tui_source_window_base (enum tui_win_type type);
 
@@ -106,8 +109,6 @@ public:
 
   virtual bool location_matches_p (struct bp_location *loc, int line_no) = 0;
 
-  void show_source_content ();
-
   void update_exec_info ();
 
   /* Update the window to display the given location.  Does nothing if