Handle fixed-point division by zero

Handle fixed-point division by zero

fixed_point_binop did not account for division by zero.  This would
lead to gdb getting SIGFPE and subsequently cause some test cases to
hang.

gdb/ChangeLog
2020-12-14  Tom Tromey  <tromey@adacore.com>

	* valarith.c (fixed_point_binop): Call error on division by zero.

gdb/testsuite/ChangeLog
2020-12-14  Tom Tromey  <tromey@adacore.com>

	* gdb.dwarf2/dw2-fixed-point.exp: Add test for division by zero.