misc: chore: Fix numerous NullReferenceExceptions, InvalidOperationExceptions

src/Ryujinx.Cpu/Jit/HostTracked/AddressSpacePartition.cs

@@ -388,7 +388,12 @@ namespace Ryujinx.Cpu.Jit.HostTracked
                 _treeLock.ExitReadLock();
             }
 
-            return (_backingMemory, _firstPagePa.Value);
+            if (_firstPagePa != null)
+            {
+                return (_backingMemory, _firstPagePa.Value);
+            }
+
+            return (null, 0);
         }
 
         public PrivateRange GetPrivateAllocation(ulong va)

src/Ryujinx.Cpu/Jit/MemoryManagerHostTracked.cs

@@ -135,7 +135,10 @@ namespace Ryujinx.Cpu.Jit
                     offset += (int)copySize;
                 }
 
-                return new ReadOnlySequence<byte>(first, 0, last, (int)(size - last.RunningIndex));
+                if (last != null)
+                {
+                    return new ReadOnlySequence<byte>(first, 0, last, (int)(size - last.RunningIndex));
+                }
             }
             catch (InvalidMemoryRegionException)
             {
@@ -146,6 +149,8 @@ namespace Ryujinx.Cpu.Jit
 
                 return ReadOnlySequence<byte>.Empty;
             }
+
+            return default;
         }
 
         /// <inheritdoc/>