When extracting curly braces (e.g. the "upstream" part in
"HEAD@{upstream}"), we put the curly braces' contents into a `git_buf`
structure, but don't check the return value of `git_buf_putc`. So when
we run out-of-memory, we'll use a partially filled buffer without
noticing.

Let's fix this issue by checking `git_buf_putc`'s return value.