target-mips: fix corner case in TLBWR causing QEMU to hang

cpu_mips_get_random() function is used to generate a random index from
CP0.Wired to TLBSize-1 range. Current implementation avoids generating
the same as before value, hence the while loop. If the guest sets
CP0.Wired to TLBSize-1 (which actually does not sound to be very
practical) QEMU will get stuck in the loop infinitely as we always
generate the same index.
Signed-off-by: NLeon Alrae <leon.alrae@imgtec.com>
Reviewed-by: NAurelien Jarno <aurelien@aurel32.net>

hw/mips/cputimer.c

@@ -33,13 +33,18 @@ uint32_t cpu_mips_get_random (CPUMIPSState *env)
     static uint32_t seed = 1;
     static uint32_t prev_idx = 0;
     uint32_t idx;
+    uint32_t nb_rand_tlb = env->tlb->nb_tlb - env->CP0_Wired;
+
+    if (nb_rand_tlb == 1) {
+        return env->tlb->nb_tlb - 1;
+    }
+
     /* Don't return same value twice, so get another value */
     do {
         /* Use a simple algorithm of Linear Congruential Generator
          * from ISO/IEC 9899 standard. */
         seed = 1103515245 * seed + 12345;
-        idx = (seed >> 16) % (env->tlb->nb_tlb - env->CP0_Wired) +
-              env->CP0_Wired;
+        idx = (seed >> 16) % nb_rand_tlb + env->CP0_Wired;
     } while (idx == prev_idx);
     prev_idx = idx;
     return idx;