swresample/resample: free existing ResampleContext on reinit

Fixes memleak.
Reviewed-by: wm4 <nfxjfg@googlemail.com>
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: James Almer <jamrial@gmail.com>
(cherry picked from commit db7a05da)

libswresample/resample.c

@@ -190,6 +190,7 @@ static ResampleContext *resample_init(ResampleContext *c, int out_rate, int in_r
     if (!c || c->phase_shift != phase_shift || c->linear!=linear || c->factor != factor
            || c->filter_length != FFMAX((int)ceil(filter_size/factor), 1) || c->format != format
            || c->filter_type != filter_type || c->kaiser_beta != kaiser_beta) {
+        resample_free(&c);
         c = av_mallocz(sizeof(*c));
         if (!c)
             return NULL;