avutil/opt: preserve fractions in set_string_number()

This avoids going over a double precission float, which may loose precission
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/opt.c

@@ -195,6 +195,15 @@ static int set_string(void *obj, const AVOption *o, const char *val, uint8_t **d
 static int set_string_number(void *obj, void *target_obj, const AVOption *o, const char *val, void *dst)
 {
     int ret = 0, notfirst = 0;
+    int num, den;
+    char c;
+
+    if (sscanf(val, "%d%*1[:/]%d%c", &num, &den, &c) == 2) {
+        if ((ret = write_number(obj, o, dst, 1, den, num)) >= 0)
+            return ret;
+        ret = 0;
+    }
+
     for (;;) {
         int i, den = 1;
         char buf[256];

tests/ref/fate/zmbv-8bit

-#tb 0: 15967/1119068
+#tb 0: 1000000/70086303
 0,          0,          0,        1,   192000, 0x5234b617
 0,          1,          1,        1,   192000, 0x5234b617
 0,          2,          2,        1,   192000, 0x5234b617