Fixed memory leak

Please read carefully and test my changes if possible. This is my first submit to open source software, so please be kind and tell me what I can do better!

---

Found memory leak (using valgrind):

    ./configure
    make all
    valgrind --leak-check ./tig

    [...]
    ==2234== HEAP SUMMARY:
    ==2234==     in use at exit: 1,749,649 bytes in 3,411 blocks
    ==2234==   total heap usage: 6,333 allocs, 2,922 frees, 4,000,732 bytes allocated
    ==2234==
    ==2234== 9 bytes in 1 blocks are definitely lost in loss record 7 of 81
    ==2234==    at 0x4C2C04B: malloc (in
    /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
    ==2234==    by 0x416553: set_environment_variable (tig.c:8181)
    ==2234==    by 0x41661F: set_int_environment_variable (tig.c:8197)
    ==2234==    by 0x4082D2: resize_display (tig.c:2503)
    ==2234==    by 0x40B298: maximize_view (tig.c:3475)
    ==2234==    by 0x40B3E7: load_view (tig.c:3508)
    ==2234==    by 0x40B59A: open_view (tig.c:3549)
    ==2234==    by 0x40BCB2: view_driver (tig.c:3753)
    ==2234==    by 0x417D79: main (tig.c:8702)
    [...]

And fixed.

As I'm not used to the code, let me assume:

    The 'env' variable is supposed to be local. As I cannot see any
    return or arguments which interact with this variable, I think
    this variable is supposed to be local only.

I removed the malloc() call, as if the variable is local, we don't need
it. It was replaced with

    char env[len]

which gets allocated on stack. Then, I removed the check in the if

    if (env && ...

as we don't need to check if it gets allocated. Afterwards we need to
pass it to string_nformat(), which must be done with the address of the
variable, therefor I changed it (and the putenv() call).

Last but not least, I removed the

    free(env)
    return ...

lines as we can easily return the

    string_nformat(...) && putenv(env) == 0

which was the condition of the check before.

Recompiled and checked with

    make tig.o
    make all
    valgrind --leak-check=full ./tig

gives

    [...]
    ==13247== HEAP SUMMARY:
    ==13247==     in use at exit: 1,749,856 bytes in 3,411 blocks
    ==13247==   total heap usage: 6,335 allocs, 2,924 frees, 4,002,371 bytes
    allocated
    ==13247==
    ==13247== LEAK SUMMARY:
    ==13247==    definitely lost: 0 bytes in 0 blocks
    ==13247==    indirectly lost: 0 bytes in 0 blocks
    ==13247==      possibly lost: 0 bytes in 0 blocks
    ==13247==    still reachable: 1,749,856 bytes in 3,411 blocks
    ==13247==         suppressed: 0 bytes in 0 blocks
    [...]

Issues seem to be fixed now!