avutil/avstring: Fix bug and undefined behavior in av_strncasecmp()

The function in case of n=0 would read more bytes than 0.
The end pointer could be beyond the allocated space, which
is undefined.

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 6f0e9a8634)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavutil/avstring.c

@@ -222,12 +222,13 @@ int av_strcasecmp(const char *a, const char *b)
 
 int av_strncasecmp(const char *a, const char *b, size_t n)
 {
-    const char *end = a + n;
     uint8_t c1, c2;
+    if (n <= 0)
+        return 0;
     do {
         c1 = av_tolower(*a++);
         c2 = av_tolower(*b++);
-    } while (a < end && c1 && c1 == c2);
+    } while (--n && c1 && c1 == c2);
     return c1 - c2;
 }