Java front-end: fix vector index type in populate_live_range_holes

[tautschnig]

CodeQL rightly complained that there is comparison using a more narrow
type on the left-hand side of a less-than in a loop condition, which may
give rise to a non-terminating loop in case of integer overflow.

While at it, also add checks before unconditionally accessing the first
element.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a 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).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Merging #6682 (1d5c4b1) into develop (05e1fd6) will increase coverage by 0.03%.
The diff coverage is 86.68%.

@@             Coverage Diff             @@
##           develop    #6682      +/-   ##
===========================================
+ Coverage    76.89%   76.92%   +0.03%     
===========================================
  Files         1582     1583       +1     
  Lines       183170   183314     +144     
===========================================
+ Hits        140840   141018     +178     
+ Misses       42330    42296      -34     

Impacted Files	Coverage Δ
jbmc/src/java_bytecode/lazy_goto_model.cpp	82.17% <0.00%> (ø)
src/ansi-c/expr2c_class.h	100.00% <ø> (ø)
src/crangler/ctokenit.cpp	0.00% <0.00%> (ø)
src/crangler/mini_c_parser.cpp	72.67% <0.00%> (ø)
src/goto-checker/solver_factory.h	100.00% <ø> (ø)
src/goto-instrument/reachability_slicer_class.h	100.00% <ø> (ø)
src/goto-programs/goto_trace.h	100.00% <ø> (ø)
src/memory-analyzer/analyze_symbol.cpp	72.98% <0.00%> (ø)
src/symtab2gb/symtab2gb_parse_options.cpp	63.38% <0.00%> (ø)
src/util/exception_utils.cpp	91.66% <0.00%> (-6.12%)	⬇️
... and 31 more

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