analyzer: fix apparent hang with -fanalyzer-verbosity=0 [PR analyzer/99886]

analyzer: fix apparent hang with -fanalyzer-verbosity=0 [PR analyzer/99886]

The analyzer appeared to enter an infinite loop on malloc-1.c
when -fanalyzer-verbosity=0 was used.  In fact, it was slowly
counting from 0 to 0xffffffff.

Root cause is looping up to effectively ((unsigned)0) - 1 in
diagnostic_manager::consolidate_conditions when there are no events
in the path.

Fixed by the following, which uses signed integers when subtracting
from path->num_events () when simplifying checker_paths.

gcc/analyzer/ChangeLog:
	PR analyzer/99886
	* diagnostic-manager.cc
	(diagnostic_manager::prune_interproc_events): Use signed integers
	when subtracting one from path->num_events ().
	(diagnostic_manager::consolidate_conditions): Likewise.  Convert
	next_idx to a signed int.

gcc/testsuite/ChangeLog:
	PR analyzer/99886
	* gcc.dg/analyzer/pr99886.c: New test.