Fix small number rounding with fixed precision in grisu

include/fmt/format-inl.h

@@ -604,6 +604,7 @@ struct fixed_handler {
     // Check if precision is satisfied just by leading zeros, e.g.
     // format("{:.2f}", 0.001) gives "0.00" without generating any digits.
     if (precision > 0) return digits::more;
+    if (precision < 0) return digits::done;
     auto dir = get_round_direction(divisor, remainder, error);
     if (dir == unknown) return digits::error;
     buf[size++] = dir == up ? '1' : '0';

test/grisu-test.cc

@@ -40,6 +40,7 @@ TEST(GrisuTest, Zero) { EXPECT_EQ("0.0", fmt::format("{}", 0.0)); }
 TEST(GrisuTest, Round) {
   EXPECT_EQ("1.9156918820264798e-56",
             fmt::format("{}", 1.9156918820264798e-56));
+  EXPECT_EQ("0.0000", fmt::format("{:.4f}", 7.2809479766055470e-15));
 }
 
 TEST(GrisuTest, Prettify) {