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, 8 insertions, 9 deletions

diff --git a/gdb/gdb_bfd.h b/gdb/gdb_bfd.h
index ea3fdeb..2e4ac2c 100644
--- a/gdb/gdb_bfd.h
+++ b/gdb/gdb_bfd.h
@@ -115,15 +115,14 @@ void gdb_bfd_mark_parent (bfd *child, bfd *parent);
 
 void gdb_bfd_record_inclusion (bfd *includer, bfd *includee);
 
-/* Try to read or map the contents of the section SECT.  If
-   successful, the section data is returned and *SIZE is set to the
-   size of the section data; this may not be the same as the size
-   according to bfd_get_section_size if the section was compressed.
-   The returned section data is associated with the BFD and will be
-   destroyed when the BFD is destroyed.  There is no other way to free
-   it; for temporary uses of section data, see
-   bfd_malloc_and_get_section.  SECT may not have relocations.  This
-   function will throw on error.  */
+/* Try to read or map the contents of the section SECT.  If successful, the
+   section data is returned and *SIZE is set to the size of the section data;
+   this may not be the same as the size according to bfd_get_section_size if the
+   section was compressed.  The returned section data is associated with the BFD
+   and will be destroyed when the BFD is destroyed.  There is no other way to
+   free it; for temporary uses of section data, see bfd_malloc_and_get_section.
+   SECT may not have relocations.  If there is an error reading the section,
+   this issues a warning, sets *SIZE to 0, and returns NULL.  */
 
 const gdb_byte *gdb_bfd_map_section (asection *section, bfd_size_type *size);