libbb: reinstate NULL check in last_char_is()

Upstream commit 79a4032ee removed the test for a NULL argument to
last_char_is().  As reported in the upstream bug tracker this can
cause tar to segfault:

   https://bugs.busybox.net/show_bug.cgi?id=13131

1 files changed, 1 insertions, 1 deletions

diff --git a/libbb/last_char_is.c b/libbb/last_char_is.c
index fba05f974..891739b5e 100644
--- a/libbb/last_char_is.c
+++ b/libbb/last_char_is.c
@@ -11,7 +11,7 @@
 /* Find out if the last character of a string matches the one given */
 char* FAST_FUNC last_char_is(const char *s, int c)
 {
-        if (!s[0])
+        if (IF_PLATFORM_MINGW32(!s ||) !s[0])
                 return NULL;
         while (s[1])
                 s++;