Fix return value of check_linear_forms

[wegank]

I got the following warning when building msolve with LLVM Clang 19 on macOS:

libtool: compile:  clang -DHAVE_CONFIG_H -I. -I../.. -mmmx -msse -msse2 -msse3 -mssse3 -msse4.1 -msse4.2 -maes -g -O2 -c libmsolve.c  -fno-common -DPIC -o .libs/libmsolve_la-libmsolve.o
In file included from libmsolve.c:35:
In file included from ./msolve.c:24:
./lifting.c:763:1: warning: non-void function does not return a value in all control paths [-Wreturn-type]
  763 | }
      | ^
1 warning generated.

This PR fixes the UB.