Fix integer underflow in buyer algorithm

[stubbornick]

Changes Proposed:

• Fixed vector out-of-bounds access in AHBot buyer functionality

Issues Addressed:

• The crash occurs due to unsafe vector access in the AHBot buyer functionality, causing a std::out_of_range exception:
  terminate called after throwing an instance of 'std::out_of_range' what(): vector::_M_range_check: __n (which is 2968787162) >= this->size() (which is 0) Caught signal 6

Tests Performed:

• Built and tested on AzerothCore commit f332b0714077d7578e9bbe18951cefb330ec5052
• Tested in-game with AHBot buyer functionality enabled
• Verified that the server no longer crashes during AHBot buy operations

How to Test the Changes:

1. Enable the AHBot module and specifically the buyer functionality.

2. Set bids per interval higher than the actual auction count for each auction house:

   .ah bidsperinterval 2 4 
   .ah bidsperinterval 6 4 
   .ah bidsperinterval 7 4 
   

3. Create test conditions:

   • Ensure there are fewer player auctions than the bidsperinterval count
   • Make sure there are no AHBot-created auctions

4. Wait for the next AHBot cycle and verify:

   • The server doesn't crash during buy operations
   • The buy operations complete successfully