Fix signed integer overflow in pixel_minimap::prepare_cache_for_updates

[hexagonrecursion]

Summary

SUMMARY: Bugfixes "fix an integer overflow in pixel_minimap::prepare_cache_for_updates"

Purpose of change

Fixes #36807 - Initial value of cached_center_sm was causing an integer overflow

Describe the solution

1. cached_center_sm is only used in prepare_cache_for_updates
2. The first time prepare_cache_for_updates is called the cache is empty, which means both branches of if are no-op and it does not matter which one it executed.
   

   Cataclysm-DDA/src/pixel_minimap.cpp

   Lines 227 to 235 in 397d11e

   	if( std::abs( center_sm_diff.x ) > 1 ||
   	std::abs( center_sm_diff.y ) > 1 ||
   	std::abs( center_sm_diff.z ) > 0 ) {
   	cache.clear();
   	} else {
   	for( auto &mcp : cache ) {
   	mcp.second.touched = false;
   	}
   	}

3. Every time prepare_cache_for_updates is executed it updates the value of cached_center_sm

Therefore any initial value that does not cause an overflow is fine.
I think tripoint_zero will work here.

Describe alternatives you've considered

Testing

Additional context