leastsq: Fix possible return values
leastsq: Fix possible return values

According to the fortran code in lmdif.f, lmder.f and the C-wrapper in
__minpack.h, possible values for the `info` variable, which is used in
minpack.py:leastsq, are [-1, 0, 1, 2, 3, 4, 5, 6, 7, 8].

`info` in the fortran source can only be set to -1 if an exception was
raised by the objective function `func`. In this case the C-wrapper in
__minpack.h returns NULL. Hence `info` in minpack.py:leastsq can't be -1.

anntzer/scipy,pizzathief/scipy,person142/scipy,e-q/scipy,gfyoung/scipy,ilayn/scipy,lhilt/scipy,matthew-brett/scipy,scipy/scipy,jor-/scipy,scipy/scipy,nmayorov/scipy,endolith/scipy,gertingold/scipy,mdhaber/scipy,rgommers/scipy,andyfaff/scipy,matthew-brett/scipy,andyfaff/scipy,endolith/scipy,vigna/scipy,nmayorov/scipy,arokem/scipy,vigna/scipy,aarchiba/scipy,aeklant/scipy,andyfaff/scipy,arokem/scipy,rgommers/scipy,anntzer/scipy,arokem/scipy,gertingold/scipy,perimosocordiae/scipy,nmayorov/scipy,andyfaff/scipy,gfyoung/scipy,gfyoung/scipy,gertingold/scipy,ilayn/scipy,WarrenWeckesser/scipy,WarrenWeckesser/scipy,pizzathief/scipy,andyfaff/scipy,grlee77/scipy,jor-/scipy,zerothi/scipy,anntzer/scipy,matthew-brett/scipy,matthew-brett/scipy,e-q/scipy,zerothi/scipy,zerothi/scipy,person142/scipy,zerothi/scipy,ilayn/scipy,ilayn/scipy,gfyoung/scipy,pizzathief/scipy,aeklant/scipy,mdhaber/scipy,grlee77/scipy,WarrenWeckesser/scipy,anntzer/scipy,perimosocordiae/scipy,lhilt/scipy,aarchiba/scipy,perimosocordiae/scipy,jamestwebber/scipy,tylerjereddy/scipy,Stefan-Endres/scipy,grlee77/scipy,Eric89GXL/scipy,aeklant/scipy,tylerjereddy/scipy,tylerjereddy/scipy,aarchiba/scipy,nmayorov/scipy,Eric89GXL/scipy,aeklant/scipy,vigna/scipy,pizzathief/scipy,tylerjereddy/scipy,e-q/scipy,rgommers/scipy,gfyoung/scipy,jor-/scipy,vigna/scipy,Eric89GXL/scipy,matthew-brett/scipy,lhilt/scipy,aarchiba/scipy,rgommers/scipy,e-q/scipy,WarrenWeckesser/scipy,rgommers/scipy,Eric89GXL/scipy,aeklant/scipy,Eric89GXL/scipy,Stefan-Endres/scipy,ilayn/scipy,Stefan-Endres/scipy,ilayn/scipy,Stefan-Endres/scipy,jor-/scipy,endolith/scipy,person142/scipy,anntzer/scipy,arokem/scipy,endolith/scipy,tylerjereddy/scipy,perimosocordiae/scipy,mdhaber/scipy,mdhaber/scipy,nmayorov/scipy,scipy/scipy,Stefan-Endres/scipy,person142/scipy,jamestwebber/scipy,mdhaber/scipy,Eric89GXL/scipy,zerothi/scipy,arokem/scipy,anntzer/scipy,gertingold/scipy,scipy/scipy,Stefan-Endres/scipy,perimosocordiae/scipy,scipy/scipy,jamestwebber/scipy,aarchiba/scipy,grlee77/scipy,vigna/scipy,grlee77/scipy,pizzathief/scipy,endolith/scipy,endolith/scipy,WarrenWeckesser/scipy,lhilt/scipy,zerothi/scipy,e-q/scipy,jamestwebber/scipy,mdhaber/scipy,jor-/scipy,scipy/scipy,perimosocordiae/scipy,jamestwebber/scipy,WarrenWeckesser/scipy,lhilt/scipy,gertingold/scipy,person142/scipy,andyfaff/scipy