Prevent allocations of size 0 in sh_init, which are not possible with the default...

Prevent allocations of size 0 in sh_init, which are not possible with the default OPENSSL_zalloc, but are possible if the user has installed their own allocator using CRYPTO_set_mem_functions. If the 0-allocations succeeds, the secure heap code will later access (at least) the first byte of that space, which is technically an OOB access. This could lead to problems with some custom allocators that only return a valid pointer for subsequent free()-ing, and do not expect that the pointer is actually dereferenced.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2605)