assert in MetaspaceShared::map_archives

Specifying both base and dynamic archives during runtime could result in the following assert if the base archive wasn't the one used for creating the dynamic archive.

#
# A fatal error has been detected by the Java Runtime Environment:
#
# Internal Error (/ws/curr_jdk/open/src/hotspot/share/memory/metaspaceShared.cpp:2165), pid=21016, tid=21017
# assert(static_mapinfo->mapping_end_offset() == dynamic_mapinfo->mapping_base_offset()) failed: no gap
#
# JRE version: (15.0) (fastdebug build )
# Java VM: Java HotSpot(TM) 64-Bit Server VM (fastdebug 15-internal+0-2020-04-27-2351581.cccheung..., mixed mode, sharing, tiered, compressed oops, g1 gc, linux-amd64)
# Problematic frame:
# V [libjvm.so+0x129e4b3] MetaspaceShared::map_archives(FileMapInfo*, FileMapInfo*, bool)+0x83

Steps to reproduce:
(Create a jar with Hello.class)
jar cvf hello.jar Hello.class
(Create a class list)
java -cp hello.jar -XX:DumpLoadedClassList=mylist Hello
(Create a base archive using the class list)
java -cp hello.jar -Xshare:dump -XX:SharedArchiveFile=base.jsa -XX:ExtraSharedClassListFile=mylist Hello
(Create a dynamic archive)
java -cp hello.jar -XX:ArchiveClassesAtExit=dyn.jsa Hello

Runtime
-----------
(Specify dynamic archive only, passed)
java -XX:SharedArchiveFile=dyn.jsa -cp hello.jar -Xshare:on Hello
Hello World
(Specify both base and dynamic archives, failed)
java -XX:SharedArchiveFile=base.jsa:dyn.jsa -cp hello.jar -Xshare:on Hello