Don't dereference begin() on an empty vector.

The fix is obvious and the only test case I have is horrible, so I am
not including it. The problem shows up when self-hosting clang on i386
with -new-coalescer enabled.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@164793 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/LiveInterval.cpp

@@ -427,7 +427,7 @@ void LiveInterval::join(LiveInterval &Other,
 
   // If we have to apply a mapping to our base interval assignment, rewrite it
   // now.
-  if (MustMapCurValNos) {
+  if (MustMapCurValNos && !empty()) {
     // Map the first live range.
 
     iterator OutIt = begin();