se typecast 'size_t' on 'reg', not only avoid the related warning, but

se typecast 'size_t' on 'reg', not only avoid the related warning, but
also check whether less than zero -- for 'reg' is type 'int', and sizeof
(dwarf2_to_reg_map) is less than 0x7fff.

It is quoted in gdb_assert(), so need check 'reg' whether less than zero.
And the related warning (with '-W'):

  ../../binutils-gdb/gdb/microblaze-tdep.c:667:3: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]

ChangeLog:

 * microblaze-tdep.c (microblaze_dwarf2_reg_to_regnum): Check whether
 less tha zero in conditional expression.