Fix bugs related to setting a string splice to FREAD or FREADn.

[AliceLR]

Setting a string splice to FREAD now reads bytes to an intermediate buffer, which was required to fix some of the below bugs. Setting a string to FREAD normally or setting any string/splice to FREADn works fundamentally the same as before, except:

• Setting a string offset, limit, or splice to FREADn now respects the provided limit. If n is larger than the limit, n total bytes will still be read (up to the maximum string size); any bytes read past the limit are discarded.
• Setting a string offset, limit, or splice to FREAD now respects the provided limit and no longer pads the string with spaces in some cases. Setting any string to FREAD now reads the file until a terminator or the end of the file is found instead of stopping when the maximum string size is reached; any bytes read past the limit or maximum size are discarded.
• Setting a string splice to FREAD or FREADn with invalid offset or size values no longer causes a crash.
• Setting a string splice to FREAD with a valid offset that would write past the current length no longer causes a crash.