exynos: Fix out-of-bounds access in exynos4210_gcomp_find debug printf

One of the debug printfs in exynos4210_gcomp_find() will
access outside the 's->g_timer.reg.comp[]' array if there
was no active comparator and 'res' is -1. Add a conditional
to avoid this.

This doesn't happen in normal use because the debug printfs
are by default not compiled in.

Signed-off-by: Feng Jiang <jiangfeng@kylinos.cn>
Message-id: 20230404074506.112615-1-jiangfeng@kylinos.cn
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
[PMM: Adjusted commit message to clarify that the overrun
only happens if you've enabled debug printfs]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

hw/timer/exynos4210_mct.c

@@ -480,11 +480,14 @@ static int32_t exynos4210_gcomp_find(Exynos4210MCTState *s)
         res = min_comp_i;
     }
 
-    DPRINTF("found comparator %d: comp 0x%llx distance 0x%llx, gfrc 0x%llx\n",
-            res,
-            s->g_timer.reg.comp[res],
-            distance_min,
-            gfrc);
+    if (res >= 0) {
+        DPRINTF("found comparator %d: "
+                "comp 0x%llx distance 0x%llx, gfrc 0x%llx\n",
+                res,
+                s->g_timer.reg.comp[res],
+                distance_min,
+                gfrc);
+    }
 
     return res;
 }