ATRONIX: by Shixin Zeng - Fix a crash caused by integer underflow

index is unsigned, and could be zero, so 'index--' will underflow and
round to 0xffffffff, and cause problem in a later call to Expand_Series

I believe the idea of

    if ((REBINT)index > i) index--;

is to move the index forward because gob being inserted were in PANE,
and they can't be there twice (Detach_Gob will remove them from the
current gob). So if they were not there, "index" shouldn't be changed.

This fixes a crash in the following script:
    REBOL []

    foo: make block! []
    for i 0 15 1 [
	    txt: #"a" + i
	    append foo make gob! reduce/no-set [text: to string! txt]
    ]

    g: make gob! []
    append g foo

    g/pane: next g/pane

With this stack trace:
zsx@touchsmart-arch:~/work/r3.git/make$ R3_ALWAYS_MALLOC=1 ./r3-view-linux test-insert-gob-crash.r
=================================================================
==24248==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x60d00000c8a8 at pc 0x522243 bp 0x7fff91ba7a60 sp 0x7fff91ba7a50
WRITE of size 8 at 0x60d00000c8a8 thread T0
    0 0x522242 in Insert_Gobs ../src/core/t-gob.c:230
    1 0x5242d6 in Set_GOB_Var ../src/core/t-gob.c:421
 ...

Author: Oldes

src/core/t-gob.c

@@ -189,7 +189,7 @@ const REBCNT Gob_Flag_Words[] = {
 					}
 				}
 				Detach_Gob(VAL_GOB(val));
-				if ((REBINT)index > i) index--;
+				if (i >= 0 && (REBINT)index > i) index--;
 			}
 		}
 	}