libbacktrace: initialize DWARF 5 fields of unit

When I added the fields in 2019-12-13 I forgot to initialize them.

	* dwarf.c (build_address_map): Initialize DWARF 5 fields of unit.

Author: ianlancetaylor

dwarf.c

@@ -2554,6 +2554,9 @@ build_address_map (struct backtrace_state *state, uintptr_t base_address,
       u->comp_dir = NULL;
       u->abs_filename = NULL;
       u->lineoff = 0;
+      u->str_offsets_base = 0;
+      u->addr_base = 0;
+      u->rnglists_base = 0;
 
       /* The actual line number mappings will be read as needed.  */
       u->lines = NULL;