[UB] Fix another place where we would pass a null pointer to memcpy.

This too was found by UBSan. Down to 35 failures for me.

llvm-svn: 243932

llvm/include/llvm/ADT/StringMap.h

@@ -158,7 +158,8 @@ public:
 
     // Copy the string information.
     char *StrBuffer = const_cast<char*>(NewItem->getKeyData());
-    memcpy(StrBuffer, Key.data(), KeyLength);
+    if (KeyLength > 0)
+      memcpy(StrBuffer, Key.data(), KeyLength);
     StrBuffer[KeyLength] = 0;  // Null terminate for convenience of clients.
     return NewItem;
   }