Fix some issues discovered by static analysis

Running ninja -C _build/ scan-build, I now get:

• This appears to be a false positive (_fr_process_start clears out the exec_data->error field so the branch containing the if can only be entered when waitpid that assigns status is run):

  ../../../src/fr-process.c:851:7: warning: The left operand of '&' is a garbage value [core.UndefinedBinaryOperatorResult]
                  if (WIFEXITED (status)) {
                      ^          ~~~~~~
  /nix/store/p5vg4rdba95snvxd7p0km695jnm8vvwh-glibc-2.35-163-dev/include/stdlib.h:47:28: note: expanded from macro 'WIFEXITED'
  # define WIFEXITED(status)      __WIFEXITED (status)
                                  ^            ~~~~~~
  /nix/store/p5vg4rdba95snvxd7p0km695jnm8vvwh-glibc-2.35-163-dev/include/bits/waitstatus.h:37:30: note: expanded from macro '__WIFEXITED'
  #define __WIFEXITED(status)     (__WTERMSIG(status) == 0)
                                   ^          ~~~~~~
  /nix/store/p5vg4rdba95snvxd7p0km695jnm8vvwh-glibc-2.35-163-dev/include/bits/waitstatus.h:31:38: note: expanded from macro '__WTERMSIG'
  #define __WTERMSIG(status)      ((status) & 0x7f)
                                    ~~~~~~  ^

• Need to investigate

  ../../../src/glib-utils.c:663:26: warning: The left operand of '!=' is a garbage value [core.UndefinedBinaryOperatorResult]
          for (i = 0; patterns[i] != NULL; i++) {
                      ~~~~~~~~~~~ ^

• This is correct, we need to handle the errors as in the example:

  ../../../src/fr-archive-libarchive.c:1604:2: warning: Value stored to 'rb' is never read [deadcode.DeadStores]
          rb = archive_write_header (b, w_entry);
          ^    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  ../../../src/fr-archive-libarchive.c:1696:4: warning: Value stored to 'rb' is never read [deadcode.DeadStores]
                          rb = archive_write_finish_entry (b);
                          ^    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~