Bytes's Drop implementation can try to free a null pointer #340
Description
This program appears to try to free a null pointer.
fn main() {
let buf = BytesMut::with_capacity(1);
buf.freeze();
}#4 0x000055555555f117 in __rg_dealloc (arg0=0x0, arg1=1, arg2=1) at src/main.rs:6
#5 0x00005555555e1123 in alloc::alloc::dealloc (ptr=0x0, layout=...) at /rustc/4560ea788cb760f0a34127156c78e2552949f734/src/liballoc/alloc.rs:106
#6 0x00005555555e0cea in <alloc::alloc::Global as core::alloc::Alloc>::dealloc (self=0x7fffffffe8b0, ptr=..., layout=...) at /rustc/4560ea788cb760f0a34127156c78e2552949f734/src/liballoc/alloc.rs:177
#7 0x00005555555e1ec9 in alloc::raw_vec::RawVec<T,A>::dealloc_buffer (self=0x7fffffffe8b0) at /rustc/4560ea788cb760f0a34127156c78e2552949f734/src/liballoc/raw_vec.rs:742
#8 0x00005555555e1fdf in <alloc::raw_vec::RawVec<T,A> as core::ops::drop::Drop>::drop (self=0x7fffffffe8b0) at /rustc/4560ea788cb760f0a34127156c78e2552949f734/src/liballoc/raw_vec.rs:751
#9 0x00005555555e0b9f in core::ptr::real_drop_in_place () at /rustc/4560ea788cb760f0a34127156c78e2552949f734/src/libcore/ptr/mod.rs:175
#10 0x00005555555e0b32 in core::ptr::real_drop_in_place () at /rustc/4560ea788cb760f0a34127156c78e2552949f734/src/libcore/ptr/mod.rs:175
#11 0x00005555555e2237 in core::mem::drop (_x=<error reading variable: Cannot access memory at address 0x2>) at /rustc/4560ea788cb760f0a34127156c78e2552949f734/src/libcore/mem/mod.rs:706
#12 0x00005555555ded38 in bytes::bytes::shared_drop (data=0x7fffffffe928, ptr=0x1 <error: Cannot access memory at address 0x1>, len=0) at /usr/local/cargo/registry/src/github.com-1ecc6299db9ec823/bytes-0.5.2/src/bytes.rs:799
#13 0x000055555555ef70 in <bytes::bytes::Bytes as core::ops::drop::Drop>::drop (self=0x7fffffffe918) at /usr/local/cargo/registry/src/github.com-1ecc6299db9ec823/bytes-0.5.2/src/bytes.rs:435