Give GCC more control flow information, so it can be sure that

Give GCC more control flow information, so it can be sure that
variables aren't used uninitialized.
* error.h (scm_error, scm_syserror, scm_syserror_msg,
scm_sysmissing, scm_num_overflow, scm_out_of_range,
scm_wrong_num_args, scm_wrong_type_arg, scm_memory_error,
scm_misc_error): Tell GCC that these functions never return.
* struct.c (scm_struct_ref, scm_struct_set_x): If we can't figure
out the field type, call abort if SCM_ASSERT returns, to placate
the optimizer.
* stacks.c (scm_make_stack, scm_last_stack_frame): abort if
scm_wta ever returns.  We can't handle this case anyway, and this
gives the optimizer more information.
* unif.c (scm_uniform_vector_ref, scm_array_set_x): Abort if
scm_wta ever returns.

In some cases, the code is fine, but GCC isn't smart enough to
figure that out; this usually happens when one variable is only
initialized and used when a particular condition holds true, and
we know that condition will never change within a given invocation
of the function.  In this case, we simply initialize the variables
to placate the compiler, hopefully to a value which will cause a
crash if it is ever actually used.
* print.c (scm_iprin1): Initialize mw_pos.
* read.c (scm_lreadrecparen): Initialize tl2, ans2.
* throw.c (scm_ithrow): Initialize dynpair.
* unif.c (scm_uniform_vector_ref): Initialize cra.
* struct.c (init_struct): Initialize prot.
* mbstrings.c (scm_print_mb_symbol): Initialize mw_pos and inc.