Fix invalid AMB node handle being used when low on memory

Detail:
  s/Wimp08s - If OS_AMBControl allocate is asked for a AMB node with a size > 0, but no pages can be claimed, it frees the node and returns an invalid handle (0). Make sure that the Wimp's allocateslot routine is aware of this, so that we don't try passing that bad handle back to AMBControl later on
  s/Iconbar - Use nullptr constant instead of -1 when checking task_slotsize (aka AMB node handle on modern machines)
Admin:
  Tested on BB-xM
  Fixes abort seen when attempting to start a task with no pages in the free pool


Version 5.53. Tagged as 'Wimp-5_53'

VersionASM

@@ -11,13 +11,13 @@
                         GBLS    Module_HelpVersion
                         GBLS    Module_ComponentName
                         GBLS    Module_ComponentPath
-Module_MajorVersion     SETS    "5.52"
-Module_Version          SETA    552
+Module_MajorVersion     SETS    "5.53"
+Module_Version          SETA    553
 Module_MinorVersion     SETS    ""
-Module_Date             SETS    "08 May 2016"
-Module_ApplicationDate  SETS    "08-May-16"
+Module_Date             SETS    "23 May 2016"
+Module_ApplicationDate  SETS    "23-May-16"
 Module_ComponentName    SETS    "Wimp"
 Module_ComponentPath    SETS    "castle/RiscOS/Sources/Desktop/Wimp"
-Module_FullVersion      SETS    "5.52"
-Module_HelpVersion      SETS    "5.52 (08 May 2016)"
+Module_FullVersion      SETS    "5.53"
+Module_HelpVersion      SETS    "5.53 (23 May 2016)"
                         END

VersionNum

-/* (5.52)
+/* (5.53)
  *
  * This file is automatically maintained by srccommit, do not edit manually.
  * Last processed by srccommit version: 1.1.
  *
  */
-#define Module_MajorVersion_CMHG        5.52
+#define Module_MajorVersion_CMHG        5.53
 #define Module_MinorVersion_CMHG        
-#define Module_Date_CMHG                08 May 2016
+#define Module_Date_CMHG                23 May 2016
 
-#define Module_MajorVersion             "5.52"
-#define Module_Version                  552
+#define Module_MajorVersion             "5.53"
+#define Module_Version                  553
 #define Module_MinorVersion             ""
-#define Module_Date                     "08 May 2016"
+#define Module_Date                     "23 May 2016"
 
-#define Module_ApplicationDate          "08-May-16"
+#define Module_ApplicationDate          "23-May-16"
 
 #define Module_ComponentName            "Wimp"
 #define Module_ComponentPath            "castle/RiscOS/Sources/Desktop/Wimp"
 
-#define Module_FullVersion              "5.52"
-#define Module_HelpVersion              "5.52 (08 May 2016)"
-#define Module_LibraryVersionInfo       "5:52"
+#define Module_FullVersion              "5.53"
+#define Module_HelpVersion              "5.53 (23 May 2016)"
+#define Module_LibraryVersionInfo       "5:53"

s/Iconbar

@@ -684,7 +684,7 @@ mapslotinout
         TST     R0,#&80000000
         BNE     %FT01
         LDR     R0,[R0,#task_slotptr]
-        CMP     R0,#-1
+        CMP     R0,#nullptr
         BLNE    mapout
 01
         Pull    "R0"
@@ -695,7 +695,7 @@ mapslotinout
         TST     R0,#&80000000
         Pull    "R1-R2,PC",NE
         LDR     R0,[R0,#task_slotptr]
-        CMP     R0,#-1
+        CMP     R0,#nullptr
         BLNE    mapin
         Pull    "R1-R2,PC"
         ]

s/Wimp08s

@@ -1095,6 +1095,8 @@ allocateslot    ROUT
        BL      setdefaulthandlers
        Pull    "R0-R4,PC"
 01
+       CMP     R2,#0
+       MOVEQ   R2,#nullptr
     Debug mjs2,"  allocateslot pages,slotptr",R1,R2
        LDR     R1,taskhandle
        LDR     R1,[wsptr,R1]