pp.c:pp_negate: Move looks_like_number where it matters

pp.c:pp_negate: Move looks_like_number where it matters

Since we already have a check further down to see whether a string
begins with an identifier or sign, and since looks_like_number
was added for strings representing negative numbers, move the
looks_like_number down to where we already know the string
begins with '-'.

This is a micro-optimisation, but it also makes the code more
straightforward (to me at least).

This happens to let magical integers-as-strings fall down to code that
they used not to reach, so that has to change to account.