scan-build raises an error on sds

Question

I'm using the sds library from redis and run scan-build on it. Then I get two errors, that are very similair. I'll present one of the errors here.

sds.c:92:22: warning: Out of bound memory access (accessed memory precedes memory block)
sh->buf[initlen] = '\0';    

The function in question is:

    /* Create a new sds string with the content specified by the 'init' pointer
     * and 'initlen'.
     * If NULL is used for 'init' the string is initialized with zero bytes.
     *
     * The string is always null-termined (all the sds strings are, always) so
     * even if you create an sds string with:
     *
     * mystring = sdsnewlen("abc",3");
     *
     * You can print the string with printf() as there is an implicit \0 at the
     * end of the string. However the string is binary safe and can contain
     * \0 characters in the middle, as the length is stored in the sds header. */
    sds sdsnewlen(const void *init, size_t initlen) {
        struct sdshdr *sh;

        if (init) {
            sh = malloc(sizeof *sh+initlen+1);
        } else {
            sh = calloc(sizeof *sh+initlen+1,1);
        }
        if (sh == NULL) return NULL;
        sh->len = initlen;
        sh->free = 0;
        if (initlen && init)
            memcpy(sh->buf, init, initlen);
        sh->buf[initlen] = '\0';
        return (char*)sh->buf;
    }

I really don't understand this error and I'm pretty sure there isn't an error here since it's a quite fundamental part of the sds library that should have shown itself in production already if there was a bug.

How can I fix this? Either by explicit tell cland-analyzer that this is not a bug or by changing the code to make clang-analyzer happy.