lavf: Only initialize s->offset once when using avoid_negative_ts make_zero

When given a stream starting at dts=0, it would previously consider
s->offset as uninitialized and set an offset when the second packet
was written, ending up writing two packets with dts=0. By initializing
this field to AV_NOPTS_VALUE, we make sure that we only initialize it
once, on the first packet.

Signed-off-by: Martin Storsjö <martin@martin.st>
librtmp: append the correct field to the string

Also prevent a NULL pointer dereference.

CC: libav-stable@libav.org
Bug-Id: CID 1250329 / CID 1250331
libvpxenc: clean memory on error

CC: libav-stable@libav.org
Bug-Id: CID 733795
lavc: fix bitshifts amount bigger than the type

CC: libav-stable@libav.org
Bug-Id: CID 1194387 / CID 1194389 / CID 1194393 / CID 1206638
mov: check ff_get_wav_header() return value

CC: libav-stable@libav.org
Bug-Id: CID 717497
Merge commit '9257692ac15eff7b07540c1f61cebde0d8823fbd'

* commit '9257692ac15eff7b07540c1f61cebde0d8823fbd':
  lavf: Only initialize s->offset once when using avoid_negative_ts make_zero

Conflicts:
	libavformat/mux.c

Merged-by: Michael Niedermayer <michaelni@gmx.at>
Merge commit 'a28468d0daf4be14761c16a3ddd33266b2380123'

* commit 'a28468d0daf4be14761c16a3ddd33266b2380123':
  librtmp: append the correct field to the string

See: d1970929b5f8b873aac171586343c9d8142897ad
Merged-by: Michael Niedermayer <michaelni@gmx.at>
Merge commit '771656bd85416cd6308b11aed6f2c69a8db9c21b'

* commit '771656bd85416cd6308b11aed6f2c69a8db9c21b':
  libvpxenc: clean memory on error

Conflicts:
	libavcodec/libvpxenc.c

See: 104b1d9e103f90485e894b20dd5bb3f1964fe5f3
Merged-by: Michael Niedermayer <michaelni@gmx.at>
Merge commit '85dc006b1a829726dd5e3a9b0fcc6a1dbfe6dffa'

* commit '85dc006b1a829726dd5e3a9b0fcc6a1dbfe6dffa':
  lavc: fix bitshifts amount bigger than the type

Conflicts:
	libavcodec/internal.h

Merged-by: Michael Niedermayer <michaelni@gmx.at>
Merge commit '2007082d2db25f9305b8a345798b840ea7784fdb'

* commit '2007082d2db25f9305b8a345798b840ea7784fdb':
  mov: check ff_get_wav_header() return value

Conflicts:
	libavformat/mov.c

See: 6d55a40b00801899f7975b22401bdd96bbc228af
Merged-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/internal: Add () to argument of FF_SIGNBIT() to ensure correct order or operations

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>