Avoid possible dereference null pointer (src/backend/catalog/pg_depend.c)

Hi.

Per Coverity.

2. returned_null: SearchSysCacheAttName returns NULL (checked 20 out of 21
times).
3. var_assigned: Assigning: ptup = NULL return value from
SearchSysCacheAttName.
964 ptup = SearchSysCacheAttName(relid, attname);
CID 1545986: (#1 of 1): Dereference null return value (NULL_RETURNS)
4. dereference: Dereferencing ptup, which is known to be NULL.

The functions SearchSysCacheAttNum and SearchSysCacheAttName,
need to have the result checked.

The commit 5091995
<https://github.com/postgres/postgres/commit/509199587df73f06eda898ae13284292f4ae573a>,
left an oversight.

Fixed by the patch attached, a change of style, unfortunately, was
necessary.

best regards,
Ranier Vilela