Fix sometimes-uninitialized warning in gdbscm_value_address

Fix sometimes-uninitialized warning in gdbscm_value_address

I am getting this warning with clang:

/home/emaisin/src/binutils-gdb/gdb/guile/scm-value.c:439:11: error: variable 'address' is used uninitialized whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]
      if (res_val != NULL)
          ^~~~~~~~~~~~~~~
/home/emaisin/src/binutils-gdb/gdb/guile/scm-value.c:444:32: note: uninitialized use occurs here
      if (gdbscm_is_exception (address))
                               ^~~~~~~
/home/emaisin/src/binutils-gdb/gdb/guile/scm-value.c:439:7: note: remove the 'if' if its condition is always true
      if (res_val != NULL)
      ^~~~~~~~~~~~~~~~~~~~
/home/emaisin/src/binutils-gdb/gdb/guile/scm-value.c:427:18: note: initialize the variable 'address' to silence this warning
      SCM address;
                 ^
                  = nullptr

We can get rid of it with a small refactoring.  I think it's a bit
cleaner/safer to initialize address with a pessimistic value and assign
it on success.  Then there's no chance of using it uninitialized.  If I
understand correctly, the NULL check on res_val was to check whether
value_addr threw, and that if value_addr returns without throwing, the
result will never be NULL.  If that's true, we can skip the res_val
variable.

Tested by running gdb.guile/*.exp locally.

gdb/ChangeLog:

	* guile/scm-value.c (gdbscm_value_address): Initialize address,
	get rid of res_val.