Fix parsing of Edit option 'A'
Robert Sprowson authored
The change of MIN() to a macro in RISC_OSLib-6_02 tripped up as it called the argument number parse function twice, which advances the parsing offset as a sideeffect. Read it once, then evaluate MIN().
Mark the string argument to txtar__readoptnum() as const.
Ref: https://www.riscosopen.org/forum/forums/11/topics/14632

Version 6.03. Tagged as 'RISC_OSLib-6_03'
57282c86