Fix for access outside array bounds

Can't allocate PC, so loop can stop at R14.
Ref https://www.riscosopen.org/forum/forums/4/topics/9503

Version 1.80. Tagged as 'SprExtend-1_80'

VersionASM

@@ -11,13 +11,13 @@
                         GBLS    Module_HelpVersion
                         GBLS    Module_ComponentName
                         GBLS    Module_ComponentPath
-Module_MajorVersion     SETS    "1.79"
-Module_Version          SETA    179
+Module_MajorVersion     SETS    "1.80"
+Module_Version          SETA    180
 Module_MinorVersion     SETS    ""
-Module_Date             SETS    "28 Feb 2017"
-Module_ApplicationDate  SETS    "28-Feb-17"
+Module_Date             SETS    "22 Oct 2017"
+Module_ApplicationDate  SETS    "22-Oct-17"
 Module_ComponentName    SETS    "SprExtend"
 Module_ComponentPath    SETS    "mixed/RiscOS/Sources/Video/Render/SprExtend"
-Module_FullVersion      SETS    "1.79"
-Module_HelpVersion      SETS    "1.79 (28 Feb 2017)"
+Module_FullVersion      SETS    "1.80"
+Module_HelpVersion      SETS    "1.80 (22 Oct 2017)"
                         END

VersionNum

-/* (1.79)
+/* (1.80)
  *
  * This file is automatically maintained by srccommit, do not edit manually.
  * Last processed by srccommit version: 1.1.
  *
  */
-#define Module_MajorVersion_CMHG        1.79
+#define Module_MajorVersion_CMHG        1.80
 #define Module_MinorVersion_CMHG        
-#define Module_Date_CMHG                28 Feb 2017
+#define Module_Date_CMHG                22 Oct 2017
 
-#define Module_MajorVersion             "1.79"
-#define Module_Version                  179
+#define Module_MajorVersion             "1.80"
+#define Module_Version                  180
 #define Module_MinorVersion             ""
-#define Module_Date                     "28 Feb 2017"
+#define Module_Date                     "22 Oct 2017"
 
-#define Module_ApplicationDate          "28-Feb-17"
+#define Module_ApplicationDate          "22-Oct-17"
 
 #define Module_ComponentName            "SprExtend"
 #define Module_ComponentPath            "mixed/RiscOS/Sources/Video/Render/SprExtend"
 
-#define Module_FullVersion              "1.79"
-#define Module_HelpVersion              "1.79 (28 Feb 2017)"
-#define Module_LibraryVersionInfo       "1:79"
+#define Module_FullVersion              "1.80"
+#define Module_HelpVersion              "1.80 (22 Oct 2017)"
+#define Module_LibraryVersionInfo       "1:80"

c/asmcore

@@ -1005,7 +1005,7 @@ static void allocate_registers(asm_workspace *wp, workspace *ws)
       int blocking = (r->flags & REGFLAG_USAGE_MASK) | REGFLAG_GLOBAL;
       if(r->flags & REGFLAG_TEMPORARY)
       {
-        for(int j=15;j>=0;j--)
+        for(int j=14;j>=0;j--)
         {
           if(!usedregs[j])
           {