X86PagingStructuresPAE: Zero fPageDirPointerTable in constructor

... and use it as a guard in the destructor. Fixes crash when running
out of memory and Init() is not called.

src/system/kernel/arch/x86/paging/pae/X86PagingStructuresPAE.cpp

@@ -20,6 +20,8 @@
 
 
 X86PagingStructuresPAE::X86PagingStructuresPAE()
+	:
+	fPageDirPointerTable(NULL)
 {
 	memset(fVirtualPageDirs, 0, sizeof(fVirtualPageDirs));
 }
@@ -32,8 +34,10 @@ X86PagingStructuresPAE::~X86PagingStructuresPAE()
 		// There's one contiguous allocation for 0 and 1.
 
 	// free the PDPT page
+	if (fPageDirPointerTable != NULL) {
 		X86PagingMethodPAE::Method()->Free32BitPage(fPageDirPointerTable,
 			pgdir_phys, fPageDirPointerTableHandle);
+	}
 }