Fix build bots after r317622

Example build failure: http://lab.llvm.org:8011/builders/lld-x86_64-darwin13/builds/14660

TIL that the warning flags for local builds are loose compared to what build
servers use.

llvm-svn: 317626

llvm/tools/yaml2obj/yaml2elf.cpp

@@ -76,7 +76,7 @@ public:
   }
   /// asserts if name is not present in the map
   unsigned get(StringRef Name) const {
-    unsigned Idx;
+    unsigned Idx = 0;
     assert(!lookup(Name, Idx) && "Expected section not found in index");
     return Idx;
   }