4.3.1: -Wconversion-null warning during build

[jengelh]

Singular 4.3.1 is calling some of its own functions not properly. Warning seen on x86_64-linux target:

flintcf_Zn.cc: In function 'CanonicalForm ConvSingNFactoryN(number, BOOLEAN, coeffs)':
flintcf_Zn.cc:420:10: warning: passing NULL to non-pointer argument 1 of 'CanonicalForm::CanonicalForm(long int)' [-Wconversion-null]
  420 |   return NULL;
      |          ^~~~
In file included from flintcf_Zn.cc:15:
/home/jengelh/obs/zu/science/singular/Sources/factory/factory.h:245:30: note:   declared here
  245 |     CF_INLINE CanonicalForm( const long );
      |                              ^~~~~~~~~~