http: Properly initialize icy headers string

The icy_metadata_headers string never gets initialized, so,
during the first call to av_strlcatf() in parse_icy(),
strlen() will be called on a pointer to uninitialized memory.
At best this causes some garbage data to be left at the
start of the string.

By initializing icy_metadata_headers to the empty string, the
first call to strlen() will always return 0, so that data is
appended from the start of the string.

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
Merge remote-tracking branch 'qatar/master'

* qatar/master:
  http: Properly initialize icy headers string

Merged-by: Michael Niedermayer <michaelni@gmx.at>
mvformat/movenc: fix IMX

fixes Ticket3351

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