[ELF] - Do not segfault when using --gc-sections with linker script

Patch fixes PR32024.

Sections that were not marked as Live has null output section.
Previously we tried to access that field and segfaulted.

Differential revision: https://reviews.llvm.org/D30188

llvm-svn: 295727
diff --git a/lld/ELF/LinkerScript.cpp b/lld/ELF/LinkerScript.cpp
index db7bb03..a5ecf48 100644
--- a/lld/ELF/LinkerScript.cpp
+++ b/lld/ELF/LinkerScript.cpp
@@ -503,6 +503,8 @@
         continue;
 
     auto *IB = static_cast<InputSectionBase<ELFT> *>(ID);
+    if (!IB->Live)
+      continue;
     switchTo(IB->OutSec);
     if (auto *I = dyn_cast<InputSection<ELFT>>(IB))
       output(I);