fp_uscale.c: zero-initialize digs to silence MinGW gcc warning

MinGW gcc emits -Wmaybe-uninitialized for `digs[0]` in the
`mrb_format_float` function because it cannot prove that
`count_digits()` always returns >= 1. The code is correct
(count_digits's `d == 0` early return makes the lower bound
clear to a human reader), but gcc's flow analysis does not
follow through. Initialize `digs` to silence the false positive
across all MinGW gcc CI configurations.

Co-authored-by: Claude <noreply@anthropic.com>

src/fp_uscale.c

@@ -1204,7 +1204,7 @@ mrb_format_float(mrb_float f, char *buf, size_t buf_size, char fmt, int prec, ch
     /* nonzero finite */
     uint64_t d;
     int p, nd, exp;
-    char digs[20];
+    char digs[20] = {0};  /* gcc -Wmaybe-uninitialized false positive: count_digits() always returns >= 1 */
 
     if (fmt == 'S') {
       /* shortest representation for to_s */