fix(debug): Replace nullptr with false in DEBUG_ASSERTCRASH macros

Fix all instances of DEBUG_ASSERTCRASH(nullptr, ...) that cause compilation
errors in MinGW-w64 debug builds due to implicit nullptr-to-bool conversion.

Changes:
- PlayerTemplate.cpp: 1 instance fixed
- ChallengeGenerals.cpp: 1 instance fixed
- LoadScreen.cpp: 2 instances fixed

Total: 4 instances fixed across GeneralsMD codebase

The DEBUG_ASSERTCRASH macro expects a boolean condition as its first
argument. Using nullptr (std::nullptr_t) requires direct-initialization
to convert to bool, which MinGW treats as an error with -fpermissive.

These instances were introduced by commit f891c5f ("refactor: Modernize
NULL to nullptr") which converted NULL to nullptr throughout the codebase.
However, DEBUG_ASSERTCRASH(NULL, ...) was semantically DEBUG_ASSERTCRASH(0, ...)
meaning "unconditional crash", which is more explicitly written as
DEBUG_ASSERTCRASH(false, ...).

Error resolved:
  error: converting to 'bool' from 'std::nullptr_t' requires
  direct-initialization [-fpermissive]

VC6 compatibility: The 'false' keyword is fully supported in C++98 and
VC6, making this change safe for all target compilers.

Affects: GeneralsMD debug builds only

Author: JohnsterID

GeneralsMD/Code/GameEngine/Source/Common/RTS/PlayerTemplate.cpp

@@ -294,7 +294,7 @@ Int PlayerTemplateStore::getTemplateNumByName(AsciiString name) const
 		if (m_playerTemplates[num].getName().compareNoCase(name.str()) == 0)
 			return num;
 	}
-	DEBUG_ASSERTCRASH(nullptr, ("Template doesn't exist for given name"));
+	DEBUG_ASSERTCRASH(false, ("Template doesn't exist for given name"));
 	return -1;
 }
 

GeneralsMD/Code/GameEngine/Source/GameClient/GUI/ChallengeGenerals.cpp

@@ -133,7 +133,7 @@ const GeneralPersona* ChallengeGenerals::getPlayerGeneralByCampaignName( AsciiSt
 		if (campaignName.compareNoCase( name.str() ) == 0)
 			return &m_position[i];
 	}
-	DEBUG_ASSERTCRASH(nullptr, ("Can't find General by Campaign Name"));
+	DEBUG_ASSERTCRASH(false, ("Can't find General by Campaign Name"));
 	return nullptr;
 }
 

GeneralsMD/Code/GameEngine/Source/GameClient/GUI/LoadScreen.cpp

@@ -1308,7 +1308,7 @@ void MultiPlayerLoadScreen::init( GameInfo *game )
 		else if (pt->getName() == "FactionChina")
 			portrait = TheMappedImageCollection->findImageByName("SNFactionLogoLg_China");
 		else
-			DEBUG_ASSERTCRASH(nullptr, ("Unexpected player template"));
+			DEBUG_ASSERTCRASH(false, ("Unexpected player template"));
 
 		localName = pt->getDisplayName();
 	}
@@ -1579,7 +1579,7 @@ GameSlot *lSlot = game->getSlot(game->getLocalSlotNum());
 		else if (pt->getName() == "FactionChina")
 			portrait = TheMappedImageCollection->findImageByName("SNFactionLogo144_China");
 		else
-			DEBUG_ASSERTCRASH(nullptr, ("Unexpected player template"));
+			DEBUG_ASSERTCRASH(false, ("Unexpected player template"));
 
 		localName = pt->getDisplayName();
 	}