utils: nfnl_osf: fix snprintf -Wformat-truncation warning

utils: nfnl_osf: fix snprintf -Wformat-truncation warning

Fedora 30 uses very recent gcc (version 9.1.1 20190503 (Red Hat 9.1.1-1)),
osf produces following warnings:

-Wformat-truncation warning have been introduced in the version 7.1 of gcc.
Also, remove a unneeded address check of "tmp + 1" in nf_osf_strchr().

nfnl_osf.c: In function ‘nfnl_osf_load_fingerprints’:
nfnl_osf.c:346:33: warning: ‘%s’ directive output may be truncated writing
up to 1023 bytes into a region of size 128 [-Wformat-truncation=]
  346 |   snprintf(obuf, sizeof(obuf), "%s,", pbeg);
      |                                 ^~
nfnl_osf.c:346:3: note: ‘snprintf’ output between 2 and 1025 bytes into a
destination of size 128
  346 |   snprintf(obuf, sizeof(obuf), "%s,", pbeg);
      |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
nfnl_osf.c:354:40: warning: ‘%s’ directive output may be truncated writing
up to 1023 bytes into a region of size 32 [-Wformat-truncation=]
  354 |    snprintf(f.genre, sizeof(f.genre), "%s", pbeg);
      |                                        ^~
nfnl_osf.c:354:4: note: ‘snprintf’ output between 1 and 1024 bytes into a
destination of size 32
  354 |    snprintf(f.genre, sizeof(f.genre), "%s", pbeg);
      |    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
nfnl_osf.c:363:43: warning: ‘%s’ directive output may be truncated writing
up to 1023 bytes into a region of size 32 [-Wformat-truncation=]
  363 |   snprintf(f.version, sizeof(f.version), "%s", pbeg);
      |                                           ^~
nfnl_osf.c:363:3: note: ‘snprintf’ output between 1 and 1024 bytes into a
destination of size 32
  363 |   snprintf(f.version, sizeof(f.version), "%s", pbeg);
      |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
nfnl_osf.c:370:47: warning: ‘%s’ directive output may be truncated writing
up to 1023 bytes into a region of size 32 [-Wformat-truncation=]
  370 |       snprintf(f.subtype, sizeof(f.subtype), "%s", pbeg);
      |                                               ^~
nfnl_osf.c:370:7: note: ‘snprintf’ output between 1 and 1024 bytes into a
destination of size 32
  370 |       snprintf(f.subtype, sizeof(f.subtype), "%s", pbeg);
      |       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Fernando Fernandez Mancera <ffmancera@riseup.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>