If an URI indicated that the data protocol was in use, it would be
copied into a temporary buffer via strncpy(dst, src, strlen(src)),
thereby ensuring that the trailing \0 would not be copied, despite dst
being uninitialized. dst would then be av_strdup'ed, leading to
potential segfaults.

The solution to this is simple: Don't copy the URI in the temporary
buffer at all, instead av_strdup it directly.

This fixes a -Wstringop-truncation warning emitted by GCC 9.2.
Reviewed-by: Steven Liu <lq@chinaffmpeg.org>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>