fix vector length calculation for targets in BlindTransaction

the size of surjectionTargets and targetAssetGenerator vectors
was calculated as tx.vin.size()*3, based on the fact that for
each input there might also be up to 2 issuance pseudo-inputs,
but did not take into account that the number of auxiliary generators
may exceed the number of inputs. This fixes the size calculations
by taking into account auxiliary generators supplied beyond vin size.

src/blind.cpp

@@ -235,9 +235,6 @@ int BlindTransaction(std::vector<uint256 >& input_blinding_factors, const std::v
     assert(tx.vin.size() == input_asset_blinding_factors.size());
     assert(tx.vin.size() == input_assets.size());
     assert(tx.vin.size() == input_amounts.size());
-    if (auxiliary_generators) {
-        assert(auxiliary_generators->size() >= tx.vin.size());
-    }
 
     std::vector<unsigned char*> blindptrs;
     std::vector<const unsigned char*> assetblindptrs;
@@ -255,8 +252,16 @@ int BlindTransaction(std::vector<uint256 >& input_blinding_factors, const std::v
 
     // Needed to construct the proof itself. Generators must match final transaction to be valid
     std::vector<secp256k1_generator> targetAssetGenerators;
-    surjectionTargets.resize(tx.vin.size()*3);
-    targetAssetGenerators.resize(tx.vin.size()*3);
+
+    // maxTargets is a strict upper-bound for the size of target vectors.
+    // The vectors will be shrunk later according to final count of totalTargets
+    size_t maxTargets = tx.vin.size()*3;
+    if (auxiliary_generators) {
+        assert(auxiliary_generators->size() >= tx.vin.size());
+        maxTargets += auxiliary_generators->size() - tx.vin.size();
+    }
+    surjectionTargets.resize(maxTargets);
+    targetAssetGenerators.resize(maxTargets);
 
     // input_asset_blinding_factors is only for inputs, not for issuances(0 by def)
     // but we need to create surjection proofs against this list so we copy and insert 0's