hw/misc/stm32l4x5_rcc: Add validation for MCOPRE and MCOSEL values

This commit adds validation checks for the MCOPRE and MCOSEL values in
the rcc_update_cfgr_register function. If the MCOPRE value exceeds
0b100 or the MCOSEL value exceeds 0b111, an error is logged and the
corresponding clock mux is disabled. This helps in identifying and
handling invalid configurations in the RCC registers.

Reproducer:
cat << EOF | qemu-system-aarch64 -display \
none -machine accel=qtest, -m 512M -machine b-l475e-iot01a -qtest \
stdio
writeq 0x40021008 0xffffffff
EOF

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2356
Signed-off-by: Zheyu Ma <zheyuma97@gmail.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

hw/misc/stm32l4x5_rcc.c

@@ -543,19 +543,31 @@ static void rcc_update_cfgr_register(Stm32l4x5RccState *s)
     uint32_t val;
     /* MCOPRE */
     val = FIELD_EX32(s->cfgr, CFGR, MCOPRE);
-    assert(val <= 0b100);
-    clock_mux_set_factor(&s->clock_muxes[RCC_CLOCK_MUX_MCO],
-                         1, 1 << val);
+    if (val > 0b100) {
+        qemu_log_mask(LOG_GUEST_ERROR,
+                      "%s: Invalid MCOPRE value: 0x%"PRIx32"\n",
+                      __func__, val);
+        clock_mux_set_enable(&s->clock_muxes[RCC_CLOCK_MUX_MCO], false);
+    } else {
+        clock_mux_set_factor(&s->clock_muxes[RCC_CLOCK_MUX_MCO],
+                             1, 1 << val);
+    }
 
     /* MCOSEL */
     val = FIELD_EX32(s->cfgr, CFGR, MCOSEL);
-    assert(val <= 0b111);
-    if (val == 0) {
+    if (val > 0b111) {
+        qemu_log_mask(LOG_GUEST_ERROR,
+                      "%s: Invalid MCOSEL value: 0x%"PRIx32"\n",
+                      __func__, val);
         clock_mux_set_enable(&s->clock_muxes[RCC_CLOCK_MUX_MCO], false);
     } else {
-        clock_mux_set_enable(&s->clock_muxes[RCC_CLOCK_MUX_MCO], true);
-        clock_mux_set_source(&s->clock_muxes[RCC_CLOCK_MUX_MCO],
-                             val - 1);
+        if (val == 0) {
+            clock_mux_set_enable(&s->clock_muxes[RCC_CLOCK_MUX_MCO], false);
+        } else {
+            clock_mux_set_enable(&s->clock_muxes[RCC_CLOCK_MUX_MCO], true);
+            clock_mux_set_source(&s->clock_muxes[RCC_CLOCK_MUX_MCO],
+                                 val - 1);
+        }
     }
 
     /* STOPWUCK */