Keep an object from being deallocated while pointers to or into it are in use.
Fixes these warnings:

longrat.cc: In function 'nlMapC(snumber*, n_Procs_s*, n_Procs_s*)':
longrat.cc:599:13: warning: dangling pointer to an unnamed temporary may be used [-Wdangling-pointer=]
  599 |   e=(*f)[0]._mp_exp-size;
      |     ~~~~~~~~^~~~~~~
longrat.cc:573:40: note: unnamed temporary defined here
  573 |   mpf_t *f = ((gmp_complex*)from)->real()._mpfp();
      |              ~~~~~~~~~~~~~~~~~~~~~~~~~~^~
longrat.cc:581:8: warning: using dangling pointer 'size_113' to an unnamed temporary [-Wdangling-pointer=]
  581 |   size = (*f)[0]._mp_size;
      |   ~~~~~^~~~~~~~~~~~~~~~~~
longrat.cc:573:40: note: unnamed temporary defined here
  573 |   mpf_t *f = ((gmp_complex*)from)->real()._mpfp();
      |              ~~~~~~~~~~~~~~~~~~~~~~~~~~^~
longrat.cc:592:6: warning: dangling pointer 'qp_115' to an unnamed temporary may be used [-Wdangling-pointer=]
  592 |   qp = (*f)[0]._mp_d;
      |   ~~~^~~~~~~~~~~~~~~
longrat.cc:573:40: note: unnamed temporary defined here
  573 |   mpf_t *f = ((gmp_complex*)from)->real()._mpfp();
      |              ~~~~~~~~~~~~~~~~~~~~~~~~~~^~

--- singular-4.3.2/libpolys/coeffs/longrat.cc.orig	2024-02-01 08:40:42.664437261 -0700
+++ singular-4.3.2/libpolys/coeffs/longrat.cc	2024-02-01 08:41:54.573427454 -0700
@@ -570,7 +570,8 @@ static number nlMapC(number from, const
     return INT_TO_SR(0);
   }
 
-  mpf_t *f = ((gmp_complex*)from)->real()._mpfp();
+  gmp_float gfl = ((gmp_complex*)from)->real();
+  mpf_t *f = gfl._mpfp();
 
   number res;
   mpz_ptr dest,ndest;