[RFC] Avoid crash when calling warning too early

[Tom Tromey <tom@tromey.com>]

I happened to notice that if you pass the name of an existing file
(not a directory) as the argument to --data-directory, gdb will crash:

    $ ./gdb -nx  --data-directory  ./gdb
    ../../binutils-gdb/gdb/target.c:590:56: runtime error: member call on null pointer of type 'struct target_ops'

This happens because warning ends up calling
target_supports_terminal_ours, which calls current_top_target, which
returns nullptr this early.

This fixes the problem by handling this case specially in
target_supports_terminal_ours.

I wasn't sure whether this warranted a test case, hence the RFC.

gdb/ChangeLog
2018-10-06  Tom Tromey  <tom@tromey.com>

	* target.c (target_supports_terminal_ours): Handle case where
	current_top_target returns nullptr.
---
 gdb/ChangeLog | 5 +++++
 gdb/target.c  | 5 +++++
 2 files changed, 10 insertions(+)

diff --git a/gdb/target.c b/gdb/target.c
index 2d98954b54..a261155f29 100644
--- a/gdb/target.c
+++ b/gdb/target.c
@@ -587,6 +587,11 @@ target_terminal::info (const char *arg, int from_tty)
 int
 target_supports_terminal_ours (void)
 {
+  /* This can be called before there is any target, so we must check
+     for nullptr here.  */
+  target_ops *top = current_top_target ();
+  if (top == nullptr)
+    return false;
   return current_top_target ()->supports_terminal_ours ();
 }
 
-- 
2.17.1