BUG/MEDIUM: buffer: Make sure b_is_null handles buffers waiting for allocation.

BUG/MEDIUM: buffer: Make sure b_is_null handles buffers waiting for allocation.

In b_is_null(), make sure we return 1 if the buffer is waiting for its
allocation, as users assume there's memory allocated if b_is_null() returns
0.

The indirect impact of not having this was that htxbuf() would not match
b_is_null() for a buffer waiting for an allocation, and would thus return
the value 1 for the htx pointer, causing various crashes under low memory
condition.

Note that this patch makes gcc versions 6 and above report two null-deref
warnings in proto_htx.c since htx_is_empty() continues to check for a null
pointer without knowing that this is protected by the test on b_is_null().
This is addressed by the following patches.

This should be backported to 1.9.