libdw: Enable building with -Og.

When building with -Og gcc is unable to see that __libdw_dieabbrev ()
will initialize what the second argument points to when no error occurs
as called by dwarf_child and dwarf_getattrs. Causing an possibly
uninitialized error. Just initialize readp/die_addr to NULL, which is
the value we would return if an error occurs anyway.

https://sourceware.org/bugzilla/show_bug.cgi?id=23914

Signed-off-by: Mark Wielaard <mark@klomp.org>

libdw/ChangeLog

@@ -1,3 +1,8 @@
+2018-10-23  Mark Wielaard  <mark@klomp.org>
+
+	* dwarf_child.c (__libdw_find_attr): Initialize readp to NULL.
+	* dwarf_getattrs.c (dwarf_getattrs): Initialize die_addr to NULL.
+
 2018-10-20  Mark Wielaard  <mark@klomp.org>
 
 	* libdw.map (ELFUTILS_0.175): New section. Add dwelf_elf_begin.

libdw/dwarf_child.c

@@ -43,7 +43,7 @@ internal_function
 __libdw_find_attr (Dwarf_Die *die, unsigned int search_name,
 		   unsigned int *codep, unsigned int *formp)
 {
-  const unsigned char *readp;
+  const unsigned char *readp = NULL;
 
   /* Find the abbreviation entry.  */
   Dwarf_Abbrev *abbrevp = __libdw_dieabbrev (die, &readp);

libdw/dwarf_getattrs.c

@@ -44,7 +44,7 @@ dwarf_getattrs (Dwarf_Die *die, int (*callback) (Dwarf_Attribute *, void *),
   if (unlikely (offset == 1))
     return 1;
 
-  const unsigned char *die_addr;
+  const unsigned char *die_addr = NULL;
 
   /* Find the abbreviation entry.  */
   Dwarf_Abbrev *abbrevp = __libdw_dieabbrev (die, &die_addr);