gdb: Fix "info os <unknown>" command

Running `info os someUnknownOsType` is crashing when gdb is built with
-D_GLIBCXX_DEBUG:

	/usr/include/c++/5/debug/vector:439:error: attempt to
	access an element in an empty container.

In target_read_stralloc from target.c, the call to
target_read_alloc_1 can return an empty vector, we then call vector::back on
this vector, which is invalid.

This commit adds a check for emptiness before trying to call
vector::back on it. It also adds test to check for `info os <unknown>`
to return the proper error message.

This is a regression in gdb 8.2 and this patch restores the behavior of
previous versions.

gdb/ChangeLog:

	PR gdb/23974
	* target.c (target_read_stralloc): Check for empty vector.

gdb/testsuite/ChangeLog:

	PR gdb/23974
	* gdb.base/info-os.exp: Check return for unknown "info os" type.

gdb/target.c

@@ -1793,7 +1793,7 @@ target_read_stralloc (struct target_ops *ops, enum target_object object,
   if (!buf)
     return {};
 
-  if (buf->back () != '\0')
+  if (buf->empty () || buf->back () != '\0')
     buf->push_back ('\0');
 
   /* Check for embedded NUL bytes; but allow trailing NULs.  */