Appease -Werror=stringop-overflow=

Appease -Werror=stringop-overflow=

gcc 10 seems to think of assigning to an (unsigned) char
array as a stringop and demands additional space for a
terminating '\0':

In function 'ssl3_generate_key_block',
    inlined from 'ssl3_setup_key_block' at ssl/s3_enc.c:304:11:
ssl/s3_enc.c:51:20: error: writing 1 byte into a region of size 0
[-Werror=stringop-overflow=]
   51 |             buf[j] = c;
      |             ~~~~~~~^~~
ssl/s3_enc.c: In function 'ssl3_setup_key_block':
ssl/s3_enc.c:23:19: note: at offset 16 to object 'buf' with size 16
declared here
   23 |     unsigned char buf[16], smd[SHA_DIGEST_LENGTH];
      |                   ^~~

Signed-off-by: Patrick Steuer <patrick.steuer@de.ibm.com>
Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/12632)