Don't get stuck in an infinite loop if HMC can't find an initial point

[penelopeysm]

Quick fix for #2389.

See also TuringLang/AdvancedHMC.jl#381 for added context.

Closes #2389

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.93%. Comparing base (397d1a7) to head (6fd59ac).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2392      +/-   ##
==========================================
+ Coverage   76.81%   83.93%   +7.12%     
==========================================
  Files          22       22              
  Lines        1570     1575       +5     
==========================================
+ Hits         1206     1322     +116     
+ Misses        364      253     -111     

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[coveralls]

Pull Request Test Coverage Report for Build 11724704847

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 2 (0.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.1%) to 78.585%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/mcmc/hmc.jl	0	2	0.0%

Totals
Change from base Build 11594848843:	-0.1%
Covered Lines:	1233
Relevant Lines:	1569

---

💛 - Coveralls

[yebai]

Thanks, @penelopeysm -- it looks good to me.

[willtebbutt]

Also LGTM -- definitely a drastic improvement over hanging.