[UB] Fix yet another use of memcpy with a null pointer argument. I think

this is the last of them in my build of LLVM. Haven't tried Clang yet.

Found via UBSan.

llvm-svn: 243934

llvm/lib/Support/MemoryBuffer.cpp

@@ -57,7 +57,8 @@ void MemoryBuffer::init(const char *BufStart, const char *BufEnd,
 /// CopyStringRef - Copies contents of a StringRef into a block of memory and
 /// null-terminates it.
 static void CopyStringRef(char *Memory, StringRef Data) {
-  memcpy(Memory, Data.data(), Data.size());
+  if (!Data.empty())
+    memcpy(Memory, Data.data(), Data.size());
   Memory[Data.size()] = 0; // Null terminate string.
 }