Fix potential segfault when reporting error

When reaching to SSA phase, the "error" function would always cause
segment fault due to the internal design, which is the source index
is always going to be out of bound. In this patch, we instead use
"fatal" function to avoid this issue.

Author: ChAoSUnItY

src/ssa.c

@@ -697,7 +697,7 @@ void new_name(block_t *block, var_t **var)
 var_t *get_stack_top_subscript_var(var_t *var)
 {
     if (var->base->rename.stack_idx < 1)
-        error("Index is less than 1");
+        fatal("Index is less than 1");
 
     int sub = var->base->rename.stack[var->base->rename.stack_idx - 1];
     for (int i = 0; i < var->base->subscripts_idx; i++) {