drcachesim warmup_refs is ignored for even values

There are 2 calls to check_warmed_up() which cause double-decrementing and a failure to detect hitting zero for even values:

$ clients/bin64/drcachesim -warmup_refs 50 -sim_refs 100 -indir drmemtrace.*.dir
Cache simulation results:
Core #0 (1 thread(s))
  L1I stats:
    Warmup hits:                         0
    Warmup misses:                       0
    Hits:                               75
    Misses:                              6
    Compulsory misses:                   6
    Invalidations:                       0
    Miss rate:                        7.41%

$ clients/bin64/drcachesim -warmup_refs 51 -sim_refs 100 -indir drmemtrace.*.dir
Cache simulation results:
Core #0 (1 thread(s))
  L1I stats:
    Warmup hits:                        12
    Warmup misses:                       2
    Hits:                               64
    Misses:                              4
    Compulsory misses:                   4
    Invalidations:                       0
    Miss rate:                        5.88%