Fail read calls after exception.

[bbockelm]

If a read call was performed after XrdFile threw an exception, it
is possible there are no active sources. However, not all paths
check for the case of zero sources and may use-after-free the last
source.

This caused a deadlock at the Tier-0.

@davidlange6 @Dr15Jones @slava77

[cmsbuild]

A new Pull Request was created by @bbockelm (Brian Bockelman) for CMSSW_7_5_5_patchX.

It involves the following packages:

Utilities/XrdAdaptor

@cmsbuild, @smuzaffar, @Dr15Jones can you please review it and eventually sign? Thanks.
@Martin-Grunewald, @wddgit this is something you requested to watch as well.
@slava77, @smuzaffar you are the release manager for this.
Note that this branch is designed for requested bug fixes specific to the CMSSW_7_5_5 release.
If you wish to make a pull request for the CMSSW_7_5_X release cycle, please use the CMSSW_7_5_X branch instead

Following commands in first line of a comment are recognized

• +1|approve[d]|sign[ed]: L1/L2's to approve it
• -1|reject[ed]: L1/L2's to reject it
• assign <category>[,<category>[,...]]: L1/L2's to request signatures from other categories
• unassign <category>[,<category>[,...]]: L1/L2's to remove signatures from other categories
• hold: L1/all L2's/release manager to mark it as on hold
• unhold: L1/user who put this PR on hold
• merge: L1/release managers to merge this request
• [@cmsbuild,] please test: L1/L2 and selected users to start jenkins tests
• [@cmsbuild,] please test with cms-sw/cmsdist#<PR>: L1/L2 and selected users to start jenkins tests using externals from cmsdist

[slava77]

@bbockelm please submit to 75X as well as to 76X/80X.

[slava77]

merge