[Bug #741171] pdb crashes when enabling a non-existing breakpoint

Check the supplied breakpoint number more carefully.
(Incompatibility: before this patch, "enable -1" would enable
the last breakpoint on the list; now -1 is not a legal ID.  Not sure
anyone would ever use negative indices...)

2.2 bugfix candidate, assuming making -1 illegal isn't considered a problem.
1 file changed