target/sh4: Return error if CPUClass::get_phys_page_debug() fails

target/sh4: Return error if CPUClass::get_phys_page_debug() fails

If the get_physical_address() call fails, the SH4 get_phys_page_debug()
handler returns an uninitialized address. Instead return -1, which
correspond to "no page found" (see cpu_get_phys_page_debug() doc
string).

This fixes a warning emitted when building with CFLAGS=-O3
(using GCC 10.2.1 20201125):

  target/sh4/helper.c: In function ‘superh_cpu_get_phys_page_debug’:
  target/sh4/helper.c:446:12: warning: ‘physical’ may be used uninitialized in this function [-Wmaybe-uninitialized]
    446 |     return physical;
        |            ^~~~~~~~

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Yoshinori Sato <ysato@users.sourceforge.jp>
Message-Id: <20210505161046.1397608-1-f4bug@amsat.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>