Fix a bug in printing plain strings

The bug was that I was not taking into account that I was receiving a
char** from the vector.
diff --git a/src/bc/program.c b/src/bc/program.c
index 530ab74..6278eee 100644
--- a/src/bc/program.c
+++ b/src/bc/program.c
@@ -1860,7 +1860,7 @@
 
       case BC_INST_STR:
       {
-        const char *string;
+        const char **string;
 
         idx = bc_program_index(code, &ip->idx);
 
@@ -1868,7 +1868,9 @@
 
         string = bc_vec_item(&p->strings, idx);
 
-        pchars = fprintf(stdout, "%s", string);
+        if (!string) return BC_STATUS_EXEC_INVALID_STRING;
+
+        pchars = fprintf(stdout, "%s", *string);
         status = pchars > 0 ? BC_STATUS_SUCCESS :
                               BC_STATUS_EXEC_PRINT_ERR;