Check for invalid digits when parsing octal constants

Originally, the parser had the ability to parse octal numbers
but did not check whether the digits were valid. Therefore, this
commit improves the implementation to include the validation check.

src/parser.c

@@ -634,7 +634,10 @@ void read_numeric_param(block_t *parent, basic_block_t *bb, int is_neg)
             } while (is_hex(token[i]));
         } else { /* octal */
             do {
-                c = token[i++] - '0';
+                c = token[i++];
+                if (c > '7' || c < '0')
+                    error("Invalid numeric constant");
+                c -= '0';
                 value = (value * 8) + c;
             } while (is_digit(token[i]));
         }