Wild crashes on objects with empty mergeable strings

[mati865]

I decided to try Wild against LLD's testsuite and while there are many unrelated errors (different messages, unsupported flags...), some of them like this one are real.

The problem here is Wild is crashing when encountering empty mergeable strings: https://github.com/llvm/llvm-project/blob/main/lld/test/ELF/merge-string-empty.s

********************
FAIL: lld :: ELF/merge-string-empty.s (2926 of 3097)
******************** TEST 'lld :: ELF/merge-string-empty.s' FAILED ********************
Exit Code: 101

Command Output (stderr):
--
/home/mateusz/Projects/llvm-project/build/bin/llvm-mc -filetype=obj -triple=x86_64-pc-linux /home/mateusz/Projects/llvm-project/lld/test/ELF/merge-string-empty.s -o /home/mateusz/Projects/llvm-project/build/tools/lld/test/ELF/Output/merge-string-empty.s.tmp.o # RUN: at line 4
+ /home/mateusz/Projects/llvm-project/build/bin/llvm-mc -filetype=obj -triple=x86_64-pc-linux /home/mateusz/Projects/llvm-project/lld/test/ELF/merge-string-empty.s -o /home/mateusz/Projects/llvm-project/build/tools/lld/test/ELF/Output/merge-string-empty.s.tmp.o
/home/mateusz/Projects/wild/fakes/ld.lld /home/mateusz/Projects/llvm-project/build/tools/lld/test/ELF/Output/merge-string-empty.s.tmp.o -o /dev/null # RUN: at line 5
+ /home/mateusz/Projects/wild/fakes/ld.lld /home/mateusz/Projects/llvm-project/build/tools/lld/test/ELF/Output/merge-string-empty.s.tmp.o -o /dev/null

thread 'main' panicked at /home/mateusz/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/sharded-offset-map-0.2.0/src/lib.rs:110:9:
assertion `left != right` failed
  left: 0
 right: 0
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

--

Running testsuite itself was surprisingly easy, just apply this change: mati865/llvm-project@72ecfd7 and run WILD_FAKES_DIR=~/Projects/wild/fakes ninja check-lld.

[mati865]

Created a minimal test case if somebody wants to look into it: mati865@cc7bf8d