fusermount: Fix head-buffer-overflow in extract_x_options

fusermount: Fix head-buffer-overflow in extract_x_options

Commit 74b1df2e introduced a heap-buffer-overflow, as
allocated memory was not initialized and extract_x_options
was also not checking for the remaining buffer size.
Fix is to initialize the buffer and to also not exceed the buffer
size. Actually not exceeding buffer size is rather complex with C
and introduced quite some code changes.

Also fixed is a memory leak of allocated buffers in the commit
mentioned above.