target/i386: Avoid unreachable variable declaration in mmu_translate()

target/i386: Avoid unreachable variable declaration in mmu_translate()

Coverity complains (CID 1507880) that the declaration "int error_code;"
in mmu_translate() is unreachable code. Since this is only a declaration,
this isn't actually a bug, but:
 * it's a bear-trap for future changes, because if it was changed to
   include an initialization 'int error_code = foo;' then the
   initialization wouldn't actually happen (being dead code)
 * it's against our coding style, which wants declarations to be
   at the start of blocks
 * it means that anybody reading the code has to go and look up
   exactly what the C rules are for skipping over variable declarations
   using a goto

Move the declaration to the top of the function.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20230406155946.3362077-1-peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>