Double precision floating point number contains null character when printed

[cschreib]

The append_constexpr() overload for floating point numbers incorrectly computes the number of digits required to print the float, resulting in some characters allocated in the temporary buffer not being written over. Since the temporary buffer is zero-initialised, these characters turn into null terminators.

When printed as a std::string_view (the default in snitch), the full number will be printed, but with a non-printable null character embedded in the middle of the string. When printed as a C string, the number is truncated before the exponent.

This only happens for a specific range of exponents: e-84 to e-99.

Note: this was spotted when running the snitch float-to-string code through the dtoa-benchmark, which includes some correctness checks. For fun, here was the result:

Benchmarking randomdigit ostringstream        ... [1054.500ns, 1156.900ns]
Benchmarking randomdigit ostrstream           ... [ 937.000ns, 1008.300ns]
Benchmarking randomdigit sprintf              ... [ 614.800ns,  686.300ns]
Benchmarking randomdigit snitch               ... [ 469.100ns,  491.500ns]
Benchmarking randomdigit doubleconv           ... [  61.400ns,  108.700ns]
Benchmarking randomdigit grisu2               ... [  61.300ns,   99.600ns]
Benchmarking randomdigit fpconv               ... [  57.300ns,   90.900ns]
Benchmarking randomdigit milo                 ... [  41.000ns,   88.900ns]
Benchmarking randomdigit emyg                 ... [  30.400ns,   69.000ns]
Benchmarking randomdigit floaxie              ... [  25.000ns,   67.200ns]
Benchmarking randomdigit null                 ... [   1.300ns,    1.400ns]

The correctness checks still spotted a few roundtrip issues, which are acceptable (only last digit is incorrect):

Error: roundtrip fail 1.0251092987768899e+304 -> '1.0251092987768898e+304' -> 1.0251092987768897e+304
Error: roundtrip fail 1.0894758796890851e+192 -> '1.0894758796890850e+192' -> 1.0894758796890849e+192
Error: roundtrip fail 1.0097235421977625e-199 -> '1.0097235421977624e-199' -> 1.0097235421977624e-199
Error: roundtrip fail -1.0896486723912265e+304 -> '-1.0896486723912264e+304' -> -1.0896486723912263e+304
Error: roundtrip fail 1.0372823149670409e-286 -> '1.0372823149670408e-286' -> 1.0372823149670407e-286
Error: roundtrip fail 1.0040738707345479e-258 -> '1.0040738707345478e-258' -> 1.0040738707345477e-258
Error: roundtrip fail 1.0612733881446836e-47 -> '1.0612733881446835e-47' -> 1.0612733881446834e-47
Error: roundtrip fail -1.0359632412158929e-199 -> '-1.0359632412158928e-199' -> -1.0359632412158927e-199
Error: roundtrip fail 1.1401842972637517e-94 -> '1.1401842972637516e-94' -> 1.1401842972637515e-94