Avoid inverting not_a_offset, which is minutes::min() i.e. min of long (#390)

With two's complement you can't invert the min value with `-`

Author: DavisVaughan

src/parse.h

@@ -873,7 +873,7 @@ from_stream(std::basic_istream<CharT, Traits>& is,
                             }
                         }
                     }
-                    if (neg)
+                    if (neg && !is.fail())
                         toff = -toff;
                     checked_set(temp_offset, toff, not_a_offset, is);
                     command = nullptr;