Ignore zero pairs in address descriptors read by dwarf parser

gtests os_linux.decoder_get_source_info* fail on RISC-V platform, for RISC-V's debuginfo has (0, 0) pairs in address descriptors, which will be misread as a terminating entry.

Some examples:

```
   ... readelf results ...

  Length: 220
  Version: 2
  Offset into .debug_info: 0x756b7
  Pointer Size: 8
  Segment Size: 0

    Address Length
    00000000002d431e 0000000000001e5e
    00000000002d617c 00000000000005aa
    00000000002d6726 000000000000030e
    00000000002d6a34 00000000000000c8
    00000000002d6afc 0000000000000bfe
    00000000002d76fa 00000000000002e4
    00000000002d79de 0000000000000bfe
    00000000002d85dc 00000000000002e2
    0000000000000000 0000000000000000 <- here
    00000000002d88be 00000000000000c8
    00000000002d8986 000000000000030e
    0000000000265b3e 000000000000003a
    0000000000000000 0000000000000000
```