Fix: Correct clearing of XSLT cache entry

Fix a double free that could happen when the cache was cleared
during config reload and then subsequent XSLT requests would try
to evict the same cache item, because the item name was not set to
NULL in clear_cache_entry.

Previously this was no problem as a cache entry would only be evicted
on load of a new XSLT and that one would immediately fill in the place
of the old one.
1 job from master in 11 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
failed #305
docker alpine
build

00:00:11

 
Name Stage Failure
failed
build Build There is an unknown failure, please try again
  openssl-dev-1.0.2o-r1:
conflicts:
libressl-dev-2.7.4-r0[pc:libcrypto=1.0.2o]
libressl-dev-2.7.4-r0[pc:libssl=1.0.2o]
libressl-dev-2.7.4-r0[pc:openssl=1.0.2o]
satisfies: world[openssl-dev]
libssh2-dev-1.8.0-r3[pc:libcrypto]
libssh2-dev-1.8.0-r3[pc:libssl]
ERROR: Job failed: exit code 4