exec.c: Don't accidentally sign-extend 4-byte loads in subpage_read()

In subpage_read() we perform a load of the data into a local buffer
which we then access using ldub_p(), lduw_p(), ldl_p() or ldq_p()
depending on its size, storing the result into the uint64_t *data.
Since ldl_p() returns an 'int', this means that for the 4-byte
case we will sign-extend the data, whereas for 1 and 2 byte
reads we zero-extend it.

This ought not to matter since the caller will likely ignore values in
the high bytes of the data, but add a cast so that we're consistent.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20180611171007.4165-3-peter.maydell@linaro.org

exec.c

@@ -2747,7 +2747,7 @@ static MemTxResult subpage_read(void *opaque, hwaddr addr, uint64_t *data,
         *data = lduw_p(buf);
         return MEMTX_OK;
     case 4:
-        *data = ldl_p(buf);
+        *data = (uint32_t)ldl_p(buf);
         return MEMTX_OK;
     case 8:
         *data = ldq_p(buf);