warning: null format string in stop.c

[astamm]

gcc-ubsan and gcc-asan show

warning: null format string

occurring in stop.c specifically in function nlopt_vsprintf().

According to https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116834, the fix is to first check that the pointer to format is not null and abort if it is before calling vsnprintf().