Imagemagick/convert can be crashed by processing an empty PNG

(I'm not sure if this classifies as security issue or not, as the crash itself is due to an assertion failure, not to a buffer overflow or similar).

When executing the command:

    convert empty.png -resize 150x150 -colors 16 -colorspace RGB -quantize RGB -depth 8 -alpha remove -alpha off -format %c histogram:info:

on an empty PNG, `convert` crashes with:

    convert: ../../magick/quantum.c:216: DestroyQuantumInfo: Assertion `quantum_info != (QuantumInfo *) NULL' failed.
    Aborted (core dumped)

I've provided an empty PNG file for testing.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: imagemagick-6.q16 8:6.9.7.4+dfsg-16ubuntu6.8
ProcVersionSignature: Ubuntu 5.3.0-1017.18~18.04.1-aws 5.3.18
Uname: Linux 5.3.0-1017-aws x86_64
ApportVersion: 2.20.9-0ubuntu7.15
Architecture: amd64
Date: Fri Jul 3 11:40:12 2020
Ec2AMI: ami-0df60f5a0d9a27a14
Ec2AMIManifest: (unknown)
Ec2AvailabilityZone: eu-west-1a
Ec2InstanceType: t3.xlarge
Ec2Kernel: unavailable
Ec2Ramdisk: unavailable
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=C.UTF-8
 SHELL=/bin/bash
SourcePackage: imagemagick
UpgradeStatus: No upgrade log present (probably fresh install)