[mono][interp] Fix first arg offset computation for unoptimized newobj (#85787)

Instead of obtaining the offset directly, we were computing it as the next available offset (once the arguments were pop'ed), which was not accounting for the case where the first argument was aligned.

Author: BrzVlad

src/mono/mono/mini/interp/transform.c

@@ -6035,9 +6035,15 @@ generate_code (TransformData *td, MonoMethod *method, MonoMethodHeader *header,
 				interp_ins_set_dreg (td->last_ins, td->sp [-1].local);
 			} else if (!td->optimized) {
 				int tos = get_tos_offset (td);
-				td->sp -= csignature->param_count;
-				int param_offset = get_tos_offset (td);
-				int param_size = tos - param_offset;
+				int param_offset, param_size;
+				if (csignature->param_count) {
+					td->sp -= csignature->param_count;
+					param_offset = td->sp [0].offset;
+					param_size = tos - param_offset;
+				} else {
+					param_offset = tos;
+					param_size = 0;
+				}
 
 				td->cbb->contains_call_instruction = TRUE;
 				interp_add_ins (td, MINT_NEWOBJ_SLOW_UNOPT);