Initialize unitialized M parameter in NEGEMMAssemblyDispatch

Initialize unitialized M parameter in NEGEMMAssemblyDispatch

Resolves: COMPMID-3991

Signed-off-by: Georgios Pinitas <georgios.pinitas@arm.com>
Change-Id: I353ad84833ce25e1adadb980f46b95de63dd6c62
Reviewed-on: https://review.mlplatform.org/c/ml/ComputeLibrary/+/4522
Tested-by: Arm Jenkins <bsgcomp@arm.com>
Reviewed-by: Michele Di Giorgio <michele.digiorgio@arm.com>
Comments-Addressed: Arm Jenkins <bsgcomp@arm.com>
(cherry picked from commit 71696baa30fd2cb9ac7493937e65abfb16daa16e)
Reviewed-on: https://review.mlplatform.org/c/ml/ComputeLibrary/+/4168
Tested-by: Michele Di Giorgio <michele.digiorgio@arm.com>