SEGV in ppc64_elf_get_synthetic_symtab reading a separate debug file

SEGV in ppc64_elf_get_synthetic_symtab reading a separate debug file

The attached patch fixes the SEGV and lets GDB successfully
load all kernel modules installed by default on RHEL 7.

Valgrind on F-21 x86_64 host has shown me more clear what is the problem:

Reading symbols from /home/jkratoch/t/cordic.ko...Reading symbols from
/home/jkratoch/t/cordic.ko.debug...=================================================================
==22763==ERROR: AddressSanitizer: heap-use-after-free on address 0x6120000461c8 at pc 0x150cdbd bp 0x7fffffffc7e0 sp 0x7fffffffc7d0
READ of size 8 at 0x6120000461c8 thread T0
    #0 0x150cdbc in ppc64_elf_get_synthetic_symtab /home/jkratoch/redhat/gdb-test-asan/bfd/elf64-ppc.c:3282
    #1 0x8c5274 in elf_read_minimal_symbols /home/jkratoch/redhat/gdb-test-asan/gdb/elfread.c:1205
    #2 0x8c55e7 in elf_symfile_read /home/jkratoch/redhat/gdb-test-asan/gdb/elfread.c:1268
[...]
0x6120000461c8 is located 264 bytes inside of 288-byte region [0x6120000460c0,0x6120000461e0)
freed by thread T0 here:
    #0 0x7ffff715454f in __interceptor_free (/lib64/libasan.so.1+0x5754f)
    #1 0xde9cde in xfree common/common-utils.c:98
    #2 0x9a04f7 in do_my_cleanups common/cleanups.c:155
    #3 0x9a05d3 in do_cleanups common/cleanups.c:177
    #4 0x8c538a in elf_read_minimal_symbols /home/jkratoch/redhat/gdb-test-asan/gdb/elfread.c:1229
    #5 0x8c55e7 in elf_symfile_read /home/jkratoch/redhat/gdb-test-asan/gdb/elfread.c:1268
[...]
previously allocated by thread T0 here:
    #0 0x7ffff71547c7 in malloc (/lib64/libasan.so.1+0x577c7)
    #1 0xde9b95 in xmalloc common/common-utils.c:41
    #2 0x8c4da2 in elf_read_minimal_symbols /home/jkratoch/redhat/gdb-test-asan/gdb/elfread.c:1147
    #3 0x8c55e7 in elf_symfile_read /home/jkratoch/redhat/gdb-test-asan/gdb/elfread.c:1268
[...]
SUMMARY: AddressSanitizer: heap-use-after-free /home/jkratoch/redhat/gdb-test-asan/bfd/elf64-ppc.c:3282 ppc64_elf_get_synthetic_symtab
[...]
==22763==ABORTING

A similar case a few lines later I have fixed in 2010 by:
        https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=3f1eff0a2c7f0e7078f011f55b8e7f710aae0cc2

My testcase does not always reproduce it but at least a bit:
 * GDB without ppc64 target (even as a secondary one) is reported as "untested"
 * ASAN-built GDB with ppc64 target always crashes (and PASSes with this fix)
 * unpatched non-ASAN-built GDB with ppc64 target crashes from commandline
 * unpatched non-ASAN-built GDB with ppc64 target PASSes from runtest (?)

gdb/ChangeLog
2015-02-26  Jan Kratochvil  <jan.kratochvil@redhat.com>

	* elfread.c (elf_read_minimal_symbols): Use bfd_alloc for
	bfd_canonicalize_symtab.

gdb/testsuite/ChangeLog
2015-02-26  Jan Kratochvil  <jan.kratochvil@redhat.com>

	* gdb.arch/cordic.ko.bz2: New file.
	* gdb.arch/cordic.ko.debug.bz2: New file.
	* gdb.arch/ppc64-symtab-cordic.exp: New file.