gcd_ can print "Illegal term in SymbolNormalize" with non-symbol arguments

[tueda]

This seems a bit different from #790. The result looks good, but FORM prints the somewhat unsettling message Illegal term in SymbolNormalize twice.

Example:

Off Flint;
CF f,x,y;
L F = gcd_(x+x^2,y-x^2);
P;
.end

Result:

Illegal term in SymbolNormalize
Illegal term in SymbolNormalize

   F =
      1;

Version:

FORM 5.0.0 (Jan 27 2026, v5.0.0)
+backtrace  +flint=3.4.0  +gmp=6.3.0   -mpi    -pthreads  +zlib=1.3.1
+debugging  +float        +mpfr=4.2.2  +posix  -windows   +zstd=1.5.7
Compiler: GCC 15.2.0
Architecture: x86_64

Note: FORM 4.2.0 emits this message, while FORM 4.1 doesn't.