sprintf.c: Fix -Wmaybe-uninitialized warnings in rb_str_format

Initialize `c` and `encidx` to 0. They are assigned inside
`if (n >= 0)` and the following `if (n <= 0)` calls rb_raise, but the
compiler cannot see through the noreturn guarantee.

Author: k0kubun

sprintf.c

@@ -440,8 +440,8 @@ rb_str_format(int argc, const VALUE *argv, VALUE fmt)
             {
                 VALUE val = GETARG();
                 VALUE tmp;
-                unsigned int c;
-                int n, encidx;
+                unsigned int c = 0;
+                int n, encidx = 0;
 
                 tmp = rb_check_string_type(val);
                 if (!NIL_P(tmp)) {