dns: fix passing NULL to memcpy

app-layer-dns-common.c:401:5: warning: Null pointer passed as \
                              an argument to a 'nonnull' parameter
    memcpy(ptr, fqdn, fqdn_len);
http: clear header pointer on realloc failure

Fixes:

detect-engine-hhd.c:188:5: warning: Use of memory after it is freed
    return headers_buffer;