Merge pull request #2834 from xmcclure/inflated_method_header_leak_followup

Fix uninitialized MonoError in mono_basic_block_split

Follow-up on PR #2781 which removed a mono_error_init leading to memory unsafety. This was causing crashes when verifying assemblies in the 4.4 branch although it does not seem to have caused any known problems in master for some reason.