Fix Undefined Behavior in `check_len`

[Alexhuszagh]

Comparison between pointers that do no reference the same array (or 1-past the end of the array) is undefined behavior. Quoting the Rust documentation:

If any of the following conditions are violated, the result is Undefined Behavior:

• Both the starting and resulting pointer must be either in bounds or one byte past the end of the same allocated object.
• The computed offset, in bytes, cannot overflow an isize.
• The offset being in bounds cannot rely on “wrapping around” the address space. That is, the infinite-precision sum must fit in a usize.

Likewise, quoting the LLVM Language Reference:

This value only has defined behavior when used as an operand to the ‘indirectbr’ or ‘callbr’instruction, or for comparisons against null. Pointer equality tests between labels addresses results in undefined behavior — though, again, comparison against null is ok, and no label is equal to the null pointer. This may be passed around as an opaque pointer sized value as long as the bits are not inspected. This allows ptrtoint and arithmetic to be performed on these values so long as the original value is reconstituted before the indirectbr or callbr instruction.

Therefore, the following code is undefined behavior. This should likely justify a new version release once this is published.

fast-float-rust/src/common.rs

Lines 76 to 79 in ec1b7d4

	#[inline]
	pub fn check_len(&self, n: usize) -> bool {
	unsafe { self.ptr.add(n) <= self.end }
	}

[aldanor]

Thanks for detailed investigation and the fix - this was UB indeed (perhaps not immediately obvious).