target/arm: Rearrange disabled check for watchpoints

Coverity rightly notes that ctz32(bas) on 0 will return 32,
which makes the len calculation a BAD_SHIFT.

A value of 0 in DBGWCR<n>_EL1.BAS is reserved.  Simply move
the existing check we have for this case.

Reported-by: Coverity (CID 1421964)
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20200320160622.8040-2-richard.henderson@linaro.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

target/arm/helper.c

@@ -6340,17 +6340,18 @@ void hw_watchpoint_update(ARMCPU *cpu, int n)
         int bas = extract64(wcr, 5, 8);
         int basstart;
 
-        if (bas == 0) {
-            /* This must act as if the watchpoint is disabled */
-            return;
-        }
-
         if (extract64(wvr, 2, 1)) {
             /* Deprecated case of an only 4-aligned address. BAS[7:4] are
              * ignored, and BAS[3:0] define which bytes to watch.
              */
             bas &= 0xf;
         }
+
+        if (bas == 0) {
+            /* This must act as if the watchpoint is disabled */
+            return;
+        }
+
         /* The BAS bits are supposed to be programmed to indicate a contiguous
          * range of bytes. Otherwise it is CONSTRAINED UNPREDICTABLE whether
          * we fire for each byte in the word/doubleword addressed by the WVR.