Fix parsing of Edit option 'A'
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
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