GMemoryOutputStream: Don't return -1 as a gboolean in precondition
GMemoryOutputStream: Don't return -1 as a gboolean in precondition

Since it could confuse callers (admittedly who are already violating
a precondition).

Just spotted while adapting some bits of this code for a ssh library.

gale320/glib,lukasz-skalski/glib,mzabaluev/glib,Distrotech/glib,MathieuDuponchelle/glib,ieei/glib,cention-sany/glib,mzabaluev/glib,endlessm/glib,tamaskenez/glib,tamaskenez/glib,ieei/glib,tchakabam/glib,tchakabam/glib,endlessm/glib,tamaskenez/glib,Distrotech/glib,ieei/glib,cention-sany/glib,krichter722/glib,lukasz-skalski/glib,gale320/glib,tchakabam/glib,gale320/glib,MathieuDuponchelle/glib,endlessm/glib,krichter722/glib,gale320/glib,johne53/MB3Glib,cention-sany/glib,krichter722/glib,gale320/glib,cention-sany/glib,MathieuDuponchelle/glib,MathieuDuponchelle/glib,endlessm/glib,lukasz-skalski/glib,MathieuDuponchelle/glib,mzabaluev/glib,johne53/MB3Glib,endlessm/glib,johne53/MB3Glib,tamaskenez/glib,Distrotech/glib,tamaskenez/glib,lukasz-skalski/glib,mzabaluev/glib,tchakabam/glib,krichter722/glib,krichter722/glib,johne53/MB3Glib,johne53/MB3Glib,tchakabam/glib,mzabaluev/glib,johne53/MB3Glib,Distrotech/glib,ieei/glib,ieei/glib,cention-sany/glib,Distrotech/glib,lukasz-skalski/glib