LibC: Don't clobber errno in free().

This one is a bit mysterious. I can't find any authoritative answer on what
the correct behavior is, but it seems reasonable to me that free() doesn't
step on errno, since it returns "void" and thus the caller won't know to
inspect errno anyway.

Libraries/LibC/malloc.cpp

@@ -1,5 +1,6 @@
 #include <AK/Bitmap.h>
 #include <AK/InlineLinkedList.h>
+#include <AK/ScopedValueRollback.h>
 #include <AK/Vector.h>
 #include <LibCore/CLock.h>
 #include <assert.h>
@@ -204,6 +205,8 @@ void* malloc(size_t size)
 
 void free(void* ptr)
 {
+    ScopedValueRollback rollback(errno);
+
     if (!ptr)
         return;