avoptions: Return explicitly NAN or {0,0} if the option isn't found

This actually matches what av_get_double did earlier, the
0.0/0.0 division was intentional, for producing NAN.

Still keeping the check for the return value from
av_get_number, for clarity.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavutil/opt.c

@@ -291,7 +291,7 @@ double av_get_double(void *obj, const char *name, const AVOption **o_out)
     int den=1;
 
     if (av_get_number(obj, name, o_out, &num, &den, &intnum) < 0)
-        return -1;
+        return NAN;
     return num*intnum/den;
 }
 
@@ -302,7 +302,7 @@ AVRational av_get_q(void *obj, const char *name, const AVOption **o_out)
     int den=1;
 
     if (av_get_number(obj, name, o_out, &num, &den, &intnum) < 0)
-        return (AVRational){-1, 0};
+        return (AVRational){0, 0};
     if (num == 1.0 && (int)intnum == intnum)
         return (AVRational){intnum, den};
     else