env: avoid possible NULL pointer access

env_attr_lookup call env_attr_walk with
callback = regex_callback.

In env_attr_walk
attributes = strchr(entry_cpy, ENV_ATTR_SEP)
will return NULL if ENV_ATTR_SEP is not found.

In the aftermath regex_callback may call
strlen(attributes)
with a NULL value which will lead to a failure.

The problem was indicated by scan-clam.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>