Fix compile time warnings building the binutils with clang.

bfdI would like to fix instances of the following warning, when building
with clang with no special CFLAGS other than -g3 -O0.

/home/emaisin/src/binutils-gdb/bfd/elflink.c:5425:45: error: performing pointer arithmetic on a null pointer has undefined behavior [-Werror,-Wnull-pointer-arithmetic]
    return (struct elf_link_hash_entry *) 0 - 1;
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^

Replacing those with "(struct elf_link_hash_entry *) -1" gets rid of the
warning.  I wanted to check that it didn't change the resulting code, so
I tried to build this:

  $ cat test.c
  int *before()
  {
          return (int *) 0 - 1;
  }

  int *after()
  {
          return (int *) - 1;
  }

  $ gcc -c test.c -g
  $ objdump -d test.o

  test.o:     file format elf64-x86-64

  Disassembly of section .text:

  0000000000000000 <before>:
     0:   55                      push   %rbp
     1:   48 89 e5                mov    %rsp,%rbp
     4:   48 c7 c0 fc ff ff ff    mov    $0xfffffffffffffffc,%rax
     b:   5d                      pop    %rbp
     c:   c3                      retq

  000000000000000d <after>:
     d:   55                      push   %rbp
     e:   48 89 e5                mov    %rsp,%rbp
    11:   48 c7 c0 ff ff ff ff    mov    $0xffffffffffffffff,%rax
    18:   5d                      pop    %rbp
    19:   c3                      retq

This shows that the previous code doesn't actually return -1 as the
function documentation says, but the new one does, so it's kind of a
bugfix.

bfd	* elf64-ppc.c (ppc64_elf_archive_symbol_lookup): Avoid pointer
	arithmetic on NULL pointer.
	* elflink.c (_bfd_elf_archive_symbol_lookup,
	elf_link_add_archive_symbols): Likewise.

ld	* ldexp.c (fold_name, exp_fold_tree_1): Avoid pointer arithmetic
	on NULL pointer.

3 files changed, 10 insertions, 3 deletions

diff --git a/bfd/ChangeLog b/bfd/ChangeLog
index e71a53a..664a974 100644
--- a/bfd/ChangeLog
+++ b/bfd/ChangeLog
@@ -1,3 +1,10 @@
+2018-02-01  Simon Marchi  <simon.marchi@ericsson.com>
+
+	* elf64-ppc.c (ppc64_elf_archive_symbol_lookup): Avoid pointer
+	arithmetic on NULL pointer.
+	* elflink.c (_bfd_elf_archive_symbol_lookup,
+	elf_link_add_archive_symbols): Likewise.
+
 2018-01-31  Michael Matz  <matz@suse.de>
 
 	* elflink.c (bfd_elf_define_start_stop): Fix check of
diff --git a/bfd/elf64-ppc.c b/bfd/elf64-ppc.c
index 5cbb035..9e4a4b5 100644
--- a/bfd/elf64-ppc.c
+++ b/bfd/elf64-ppc.c
@@ -5051,7 +5051,7 @@ ppc64_elf_archive_symbol_lookup (bfd *abfd,
   len = strlen (name);
   dot_name = bfd_alloc (abfd, len + 2);
   if (dot_name == NULL)
-    return (struct elf_link_hash_entry *) 0 - 1;
+    return (struct elf_link_hash_entry *) -1;
   dot_name[0] = '.';
   memcpy (dot_name + 1, name, len + 1);
   h = _bfd_elf_archive_symbol_lookup (abfd, info, dot_name);
diff --git a/bfd/elflink.c b/bfd/elflink.c
index 3fe4555..d1eb820 100644
--- a/bfd/elflink.c
+++ b/bfd/elflink.c
@@ -5422,7 +5422,7 @@ _bfd_elf_archive_symbol_lookup (bfd *abfd,
   len = strlen (name);
   copy = (char *) bfd_alloc (abfd, len);
   if (copy == NULL)
-    return (struct elf_link_hash_entry *) 0 - 1;
+    return (struct elf_link_hash_entry *) -1;
 
   first = p - name + 1;
   memcpy (copy, name, first);
@@ -5520,7 +5520,7 @@ elf_link_add_archive_symbols (bfd *abfd, struct bfd_link_info *info)
 	    }
 
 	  h = archive_symbol_lookup (abfd, info, symdef->name);
-	  if (h == (struct elf_link_hash_entry *) 0 - 1)
+	  if (h == (struct elf_link_hash_entry *) -1)
 	    goto error_return;
 
 	  if (h == NULL)