[gdb/symtab] Throw DWARF error on out-of-bounds DW_FORM_strx

With the test-case contained in the patch, and gdb build with
-fsanitize=address we get:
...
==23678==ERROR: AddressSanitizer: heap-buffer-overflow ...^M
READ of size 1 at 0x6020000c30dc thread T3^[[1m^[[0m^M
ptype global_var^M
    #0 0x2c6a40b in bfd_getl32 bfd/libbfd.c:846^M
    #1 0x168f96c in read_str_index gdb/dwarf2/read.c:15349^M
...

The executable contains an out-of-bounds DW_FORM_strx attribute:
...
$ readelf -wi $exec
<2eb>   DW_AT_name        :readelf: Warning: string index of 1 converts to \
  an offset of 0xc which is too big for section .debug_str
 (indexed string: 0x1): <string index too big>
...
and read_str_index doesn't check for this:
...
  info_ptr = (str_offsets_section->buffer
	      + str_offsets_base
	      + str_index * offset_size);
   if (offset_size == 4)
     str_offset = bfd_get_32 (abfd, info_ptr);
...
and consequently reads out-of-bounds.

Fix this in read_str_index by checking for the out-of-bounds condition and
throwing a DWARF error:
...
(gdb) ptype global_var
DWARF Error: Offset from DW_FORM_GNU_str_index or DW_FORM_strx pointing \
  outside of .debug_str_offsets section in CU at offset 0x2d7 \
  [in module dw-form-strx-out-of-bounds]
No symbol "global_var" in current context.
(gdb)
...

Tested on x86_64-linux.

Approved-By: Tom Tromey <tom@tromey.com>