Patch goto-analyzer reachable-functions json and xml output.

[markrtuttle]

Source locations occassionally omit a line number, and printing
an empty string in the json and xml output renders the output
unparsable. This patch uses "0" in place of the empty string
in the output.

• [ x] Each commit message has a non-empty body, explaining why the change was made.
• [ x] Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• [ x] The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• My commit message includes data points confirming performance improvements (if claimed).
• [ x] My PR is restricted to a single feature or bugfix.
• [ x] White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[tautschnig]

@markrtuttle and I will co-ordinate whether to go ahead with this one or with #4879, which largely fix the same problem.

[allredj]

⚠️
This PR failed Diffblue compatibility checks (cbmc commit: 3f79983).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/120770347
Status will be re-evaluated on next push.
Common spurious failures include: the cbmc commit has disappeared in the mean time (e.g. in a force-push); the author is not in the list of contributors (e.g. first-time contributors); compatibility was already broken by an earlier merge.

[codecov-io]

Codecov Report

Merging #4947 into develop will decrease coverage by <.01%.
The diff coverage is 60%.

@@             Coverage Diff             @@
##           develop    #4947      +/-   ##
===========================================
- Coverage    69.26%   69.26%   -0.01%     
===========================================
  Files         1307     1307              
  Lines       108102   108093       -9     
===========================================
- Hits         74881    74870      -11     
- Misses       33221    33223       +2

Impacted Files	Coverage Δ
src/goto-analyzer/unreachable_instructions.cpp	84.61% <60%> (-1%)	⬇️
src/util/ssa_expr.cpp	87.5% <0%> (-0.31%)	⬇️
unit/util/ssa_expr.cpp	100% <0%> (ø)	⬆️
src/util/ssa_expr.h	100% <0%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 3ad2a27...3f79983. Read the comment docs.

[tautschnig]

@markrtuttle I'm biased, but do prefer my patch in #4879 as it factors out some of the logic to avoid code repetition.