gh-120155: Fix Coverity issue in zoneinfo load_data() (#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|