Improve accuracy of rent exemption check (#168)

* Improve accuracy of rent exemption check

* Rename dlen parameter to be more descriptive

Author: tompntn

program/src/oracle/oracle.c

@@ -37,23 +37,23 @@ static bool valid_funding_account( SolAccountInfo *ka )
 
 static bool valid_signable_account( SolParameters *prm,
                                     SolAccountInfo *ka,
-                                    uint64_t dlen )
+                                    uint64_t min_dlen )
 {
   return ka->is_signer &&
          ka->is_writable &&
          SolPubkey_same( ka->owner, prm->program_id ) &&
-         ka->data_len >= dlen &&
-         is_rent_exempt( *ka->lamports, dlen );
+         ka->data_len >= min_dlen &&
+         is_rent_exempt( *ka->lamports, ka->data_len );
 }
 
 static bool valid_writable_account( SolParameters *prm,
                                     SolAccountInfo *ka,
-                                    uint64_t dlen )
+                                    uint64_t min_dlen )
 {
   return ka->is_writable &&
          SolPubkey_same( ka->owner, prm->program_id ) &&
-         ka->data_len >= dlen &&
-         is_rent_exempt( *ka->lamports, dlen );
+         ka->data_len >= min_dlen &&
+         is_rent_exempt( *ka->lamports, ka->data_len );
 }
 
 static uint64_t init_mapping( SolParameters *prm, SolAccountInfo *ka )