RFR: JDK-8280002: jmap -histo may leak stream [v2]

[Thomas Stuefe]

> Very trivial fix to a handle/memory leak. 
> 
> JDK-8215624 added parallel heap iteration to both `jmap -histo` and `jcmd GC.class_histogram`. When called with an explicit file and an invalid argument for number of threads, it leaks the file (bit of memory and a handle).
> 
> Reproduce with:
> 
> `jmap -histo:parallel=notanumber,file=xx.txt`
> 
> Can only be reproduced with jmap. jcmd is safe, arguments are handled correctly in shared code.

Thomas Stuefe has updated the pull request incrementally with one additional commit since the last revision:

  fix copyright

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/7078/files
  - new: https://git.openjdk.java.net/jdk/pull/7078/files/4d27428d..8fd4856d

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=7078&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=7078&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7078.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7078/head:pull/7078

PR: https://git.openjdk.java.net/jdk/pull/7078