stb_sprintf: avoid left shift of negative value

fix undefined behaviour reported by UBSan:
  runtime error: left shift of negative value -9223372036854775808
and add a test case.

fixes #800

stb_sprintf.h

@@ -1696,7 +1696,7 @@ static stbsp__int32 stbsp__real_to_str(char const **start, stbsp__uint32 *len, c
 
    if (expo == 0) // is zero or denormal
    {
-      if ((bits << 1) == 0) // do zero
+      if (((stbsp__uint64) bits << 1) == 0) // do zero
       {
          *decimal_pos = 1;
          *start = out;

tests/test_sprintf.c

@@ -85,6 +85,7 @@ int main()
    CHECK2("0.00", "%.2f", 1e-4);
    CHECK2("-5.20", "%+4.2f", -5.2);
    CHECK2("0.0       ", "%-10.1f", 0.);
+   CHECK2("-0.000000", "%f", -0.);
    CHECK2("0.000001", "%f", 9.09834e-07);
 #if USE_STB  // rounding differences
    CHECK2("38685626227668133600000000.0", "%.1f", pow_2_85);