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

Version 6.03. Tagged as 'RISC_OSLib-6_03'

VersionASM

@@ -9,12 +9,12 @@
                         GBLS    Module_ApplicationDate
                         GBLS    Module_HelpVersion
                         GBLS    Module_ComponentName
-Module_MajorVersion     SETS    "6.02"
-Module_Version          SETA    602
+Module_MajorVersion     SETS    "6.03"
+Module_Version          SETA    603
 Module_MinorVersion     SETS    ""
-Module_Date             SETS    "08 Jul 2019"
-Module_ApplicationDate  SETS    "08-Jul-19"
+Module_Date             SETS    "27 Jul 2019"
+Module_ApplicationDate  SETS    "27-Jul-19"
 Module_ComponentName    SETS    "RISC_OSLib"
-Module_FullVersion      SETS    "6.02"
-Module_HelpVersion      SETS    "6.02 (08 Jul 2019)"
+Module_FullVersion      SETS    "6.03"
+Module_HelpVersion      SETS    "6.03 (27 Jul 2019)"
                         END

VersionNum

-/* (6.02)
+/* (6.03)
  *
  * This file is automatically maintained by srccommit, do not edit manually.
  *
  */
-#define Module_MajorVersion_CMHG        6.02
+#define Module_MajorVersion_CMHG        6.03
 #define Module_MinorVersion_CMHG        
-#define Module_Date_CMHG                08 Jul 2019
+#define Module_Date_CMHG                27 Jul 2019
 
-#define Module_MajorVersion             "6.02"
-#define Module_Version                  602
+#define Module_MajorVersion             "6.03"
+#define Module_Version                  603
 #define Module_MinorVersion             ""
-#define Module_Date                     "08 Jul 2019"
+#define Module_Date                     "27 Jul 2019"
 
-#define Module_ApplicationDate          "08-Jul-19"
+#define Module_ApplicationDate          "27-Jul-19"
 
 #define Module_ComponentName            "RISC_OSLib"
 
-#define Module_FullVersion              "6.02"
-#define Module_HelpVersion              "6.02 (08 Jul 2019)"
-#define Module_LibraryVersionInfo       "6:2"
+#define Module_FullVersion              "6.03"
+#define Module_HelpVersion              "6.03 (27 Jul 2019)"
+#define Module_LibraryVersionInfo       "6:3"

rlib/c/txtar

@@ -2284,7 +2284,7 @@ static void txtar__dodrag(txt t)
   };
 }
 
-static int txtar__readoptnum(char *buf, int *i)
+static int txtar__readoptnum(const char *buf, int *i)
 {
   /* read a number from the option string. */
   int result = buf[*i] - '0';
@@ -2376,7 +2376,8 @@ static void txtar__defaultoptions(txtar_options *opt) {
       case 'a':
       case 'A':
         opt->big_windows = TRUE;
-        opt->big_window_size = txt1_MIN(txtar__readoptnum(buf, &i), BIG_WINDOW_SIZE_LIMIT);
+        opt->big_window_size = txtar__readoptnum(buf, &i);
+        opt->big_window_size = txt1_MIN(opt->big_window_size, BIG_WINDOW_SIZE_LIMIT);
         break;
       case 'n':
       case 'N':