[3.12] gh-120155: Fix Coverity issue in zoneinfo load_data() (GH-120232) (#120311)

gh-120155: Fix Coverity issue in zoneinfo load_data() (GH-120232)

Declare the 'rv' varaible at the top of the load_data() function to
make sure that it's initialized before the first 'goto error' which
uses 'rv' (return rv).

Fix the Coverity issue:

Error: UNINIT (CWE-457):
Python-3.12.2/Modules/_zoneinfo.c:1233:5: skipped_decl: Jumping over declaration of ""rv"".
Python-3.12.2/Modules/_zoneinfo.c:1284:5: uninit_use: Using uninitialized value ""rv"".
  1282|       }
  1283|
  1284|->     return rv;
  1285|   }
  1286|
(cherry picked from commit b90bd3e5bbc136f53b24ee791824acd6b17e0d42)

Co-authored-by: Victor Stinner <vstinner@python.org>