[rcore] Fix FileCopy() and FileMove() reporting success when the copy failed - #6364

Since #5920, FileCopy() sets result = 0 once the destination directory exists and never resets it, so it returns 0 when the source doesn't exist or is empty, or when the destination can't be written (read-only file, a directory, ...).

FileMove() relies on that result and then only checks that the destination exists with the same size. When the destination is an existing read-only file of the same size, nothing is copied but the source file is removed.

SaveFileText("a.txt", "AAAAA");
SaveFileText("b.txt", "BBBBB");   // then mark b.txt read-only
FileMove("a.txt", "b.txt");       // returns 0, a.txt is gone, b.txt still has BBBBB

FileCopy() now returns 0 only when the data was saved, and FileMove() returns -1 when the copied file doesn't check out.