Fix vertical scrolling of TUI source window

gdb/ChangeLog:
2019-03-16  Eli Zaretskii  <eliz@gnu.org>

	* source-cache.c (source_cache::get_source_lines): Call
	find_source_lines to initialize s->nlines.  This fixes vertical
	scrolling of TUI source window when the DOWN arrow is pressed.

gdb/ChangeLog

@@ -1,3 +1,9 @@
+2019-03-16  Eli Zaretskii  <eliz@gnu.org>
+
+	* source-cache.c (source_cache::get_source_lines): Call
+	find_source_lines to initialize s->nlines.  This fixes vertical
+	scrolling of TUI source window when the DOWN arrow is pressed.
+
 2019-03-16  Philippe Waroquiers  <philippe.waroquiers@skynet.be>
 
 	* auto-load.c (_initialize_auto_load): Fix 'This options has'.

gdb/source-cache.c

@@ -200,6 +200,13 @@ source_cache::get_source_lines (struct symtab *s, int first_line,
 	  std::ifstream input (fullname);
 	  if (input.is_open ())
 	    {
+	      if (s->line_charpos == 0)
+		{
+		  scoped_fd desc = open_source_file (s);
+		  if (desc.get () < 0)
+		    return false;
+		  find_source_lines (s, desc.get ());
+		}
 	      srchilite::SourceHighlight highlighter ("esc.outlang");
 	      highlighter.setStyleFile("esc.style");