Add asserts to SmallVector so that calls to front() and back() only succeed

if the vector is not empty.  This will ensure that calls to these functions
will reference elements in the vector.

llvm-svn: 173321

include/llvm/ADT/SmallVector.h

@@ -145,16 +145,20 @@ public:
   }
 
   reference front() {
+    assert(!empty());
     return begin()[0];
   }
   const_reference front() const {
+    assert(!empty());
     return begin()[0];
   }
 
   reference back() {
+    assert(!empty());
     return end()[-1];
   }
   const_reference back() const {
+    assert(!empty());
     return end()[-1];
   }
 };