wallet2: fix infinite loop in fake out selection

The gamma picker and the caller code did not quite agree on the
number of rct outputs available for use - by one block - which
caused an infinite loop if the picker could never pick outputs
from that block but already had picked all other outputs from
previous blocks.

Also change the range to select from using code from UkoeHB.

src/wallet/wallet2.cpp

@@ -1005,7 +1005,7 @@ gamma_picker::gamma_picker(const std::vector<uint64_t> &rct_offsets, double shap
   const size_t blocks_to_consider = std::min<size_t>(rct_offsets.size(), blocks_in_a_year);
   const size_t outputs_to_consider = rct_offsets.back() - (blocks_to_consider < rct_offsets.size() ? rct_offsets[rct_offsets.size() - blocks_to_consider - 1] : 0);
   begin = rct_offsets.data();
-  end = rct_offsets.data() + rct_offsets.size() - CRYPTONOTE_DEFAULT_TX_SPENDABLE_AGE;
+  end = rct_offsets.data() + rct_offsets.size() - (std::max(1, CRYPTONOTE_DEFAULT_TX_SPENDABLE_AGE) - 1);
   num_rct_outputs = *(end - 1);
   THROW_WALLET_EXCEPTION_IF(num_rct_outputs == 0, error::wallet_internal_error, "No rct outputs");
   average_output_time = DIFFICULTY_TARGET_V2 * blocks_to_consider / static_cast<double>(outputs_to_consider); // this assumes constant target over the whole rct range
@@ -8408,7 +8408,7 @@ void wallet2::get_outs(std::vector<std::vector<tools::wallet2::get_outs_entry>>
       else
       {
         // the base offset of the first rct output in the first unlocked block (or the one to be if there's none)
-        num_outs = rct_offsets[rct_offsets.size() - CRYPTONOTE_DEFAULT_TX_SPENDABLE_AGE];
+        num_outs = gamma->get_num_rct_outs();
         LOG_PRINT_L1("" << num_outs << " unlocked rct outputs");
         THROW_WALLET_EXCEPTION_IF(num_outs == 0, error::wallet_internal_error,
             "histogram reports no unlocked rct outputs, not even ours");
@@ -8692,7 +8692,7 @@ void wallet2::get_outs(std::vector<std::vector<tools::wallet2::get_outs_entry>>
       }
       bool use_histogram = amount != 0;
       if (!use_histogram)
-        num_outs = rct_offsets[rct_offsets.size() - CRYPTONOTE_DEFAULT_TX_SPENDABLE_AGE];
+        num_outs = gamma->get_num_rct_outs();
 
       // make sure the real outputs we asked for are really included, along
       // with the correct key and mask: this guards against an active attack

src/wallet/wallet2.h

@@ -101,6 +101,7 @@ namespace tools
     uint64_t pick();
     gamma_picker(const std::vector<uint64_t> &rct_offsets);
     gamma_picker(const std::vector<uint64_t> &rct_offsets, double shape, double scale);
+    uint64_t get_num_rct_outs() const { return num_rct_outputs; }
 
   private:
     struct gamma_engine