Ignore 0,0 entries in .debug_aranges

Ignore 0,0 entries in .debug_aranges

When running the internal AdaCore test suite against the new DWARF
indexer, I found one regression on RISC-V.  The test in question uses
--gc-sections, and winds up with an entry in the middle of a
.debug_aranges that has both address and length of 0.  In this
scenario, gdb assumes the entries are terminated and then proceeds to
reject the section because it reads a subsequent entry as if it were a
header.

It seems to me that, because each header describes the size of each
.debug_aranges CU, it's better to simply ignore 0,0 entries and simply
read to the end.  That is what this patch does.

I've patched an existing test to provide a regression test for this.