[ELF] - Don't segfault when accessing location counter inside MEMORY command.

We would previously crash on next script:
MEMORY { name : ORIGIN = .; }

Patch fixes that.

Differential revision: https://reviews.llvm.org/D36138

llvm-svn: 311073
diff --git a/lld/ELF/LinkerScript.cpp b/lld/ELF/LinkerScript.cpp
index 805d50b..c146c46 100644
--- a/lld/ELF/LinkerScript.cpp
+++ b/lld/ELF/LinkerScript.cpp
@@ -842,8 +842,13 @@
 }
 
 ExprValue LinkerScript::getSymbolValue(const Twine &Loc, StringRef S) {
-  if (S == ".")
-    return {CurAddressState->OutSec, Dot - CurAddressState->OutSec->Addr, Loc};
+  if (S == ".") {
+    if (CurAddressState)
+      return {CurAddressState->OutSec, Dot - CurAddressState->OutSec->Addr,
+              Loc};
+    error(Loc + ": unable to get location counter value");
+    return 0;
+  }
   if (SymbolBody *B = Symtab->find(S)) {
     if (auto *D = dyn_cast<DefinedRegular>(B))
       return {D->Section, D->Value, Loc};