Fix gdb segv when objfile can't be opened

This fixes PR 16577.

This patch changes gdb_bfd_map_section to issue a warning rather than an error
if it is unable to read the object file, and sets the size of the section/frame
that it attempted to read to 0 on error.

The description of gdb_bfd_map_section states that it will try to read or map
the contents of the section SECT, and if successful, the section data is
returned and *SIZE is set to the size of the section data.  This function was
throwing an error and leaving *SIZE as-is.  Setting the section size to 0
indicates to dwarf2_build_frame_info that there is no data to read, otherwise
it will try to read from an invalid frame pointer.

Changing the error to a warning allows this to be handled gracefully.
Additionally, the error was clobbering the breakpoint output indicating the
current frame (function name, arguments, source file, and line number).  E.g.

Thread 3 "foo" hit Breakpoint 1, BFD: reopening /tmp/jna-1013829440/jna2973250704389291330.tmp: No such file or directory
BFD: reopening /tmp/jna-1013829440/jna2973250704389291330.tmp: No such file or directory
(gdb)

While the "BFD: reopening ..." messages will still appear interspersed in the
breakpoint output, the current frame info is now displayed:

Thread 3 "foo" hit Breakpoint 1, BFD: reopening /tmp/jna-1013829440/jna1875755897659885075.tmp: No such file or directory
BFD: reopening /tmp/jna-1013829440/jna1875755897659885075.tmp: No such file or directory
warning: Can't read data for section '.eh_frame' in file '/tmp/jna-1013829440/jna1875755897659885075.tmp'
do_something () at file.cpp:80
80	{
(gdb)

1 files changed, 75 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.base/solib-vanish-main.c b/gdb/testsuite/gdb.base/solib-vanish-main.c
new file mode 100644
index 0000000..a00ee17
--- /dev/null
+++ b/gdb/testsuite/gdb.base/solib-vanish-main.c
@@ -0,0 +1,75 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2017-2018 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#include <dlfcn.h>
+#include <errno.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+int
+main ()
+{
+  void *handle;
+  int (*foo) (int);
+  char *error;
+  char *dest = VANISH_LIB ".renamed";
+
+  /* Open library.  */
+  handle = dlopen (VANISH_LIB, RTLD_NOW);
+  if (!handle)
+    {
+      fprintf (stderr, "%s\n", dlerror ());
+      exit (EXIT_FAILURE);
+    }
+
+  /* Clear any existing error */
+  dlerror ();
+
+  /* Simulate deleting file by renaming it.  */
+  if (rename (VANISH_LIB, dest) == -1)
+    {
+      error = strerror (errno);
+      fprintf (stderr, "rename %s -> %s: %s\n", VANISH_LIB, dest, error);
+      exit (EXIT_FAILURE);
+    }
+
+  /* Get function pointer.  */
+  foo = dlsym (handle, "foo");
+  error = dlerror ();
+  if (error != NULL)
+    {
+      fprintf (stderr, "%s\n", error);
+      exit (EXIT_FAILURE);
+    }
+
+  /* Call function.  */
+  (*foo) (1);
+
+  /* Close and exit.  */
+  dlclose (handle);
+
+  /* Put VANISH_LIB back where we found it.  */
+  if (rename (dest, VANISH_LIB) == -1)
+    {
+      error = strerror (errno);
+      fprintf (stderr, "rename %s -> %s: %s\n", dest, VANISH_LIB, error);
+      exit (EXIT_FAILURE);
+    }
+
+  exit (EXIT_SUCCESS);
+}