Diagnose an overall recursion (?0) inside a lookbehind as an error.

ChangeLog

@@ -96,6 +96,9 @@ only when PCRE2_NO_START_OPTIMIZE was *not* set:
 16. The "offset" modifier in pcre2test was not being ignored (as documented)
 when the POSIX API was in use.
 
+17. An overall recursion such as (?0) inside a lookbehind assertion was not 
+being diagnosed as an error.
+
 
 Version 10.22 29-July-2016
 --------------------------

src/pcre2_compile.c

@@ -8497,12 +8497,13 @@ for (;; pptr++)
       *errcodeptr = ERR15;  /* Non-existent subpattern */
       return -1;
       }
-
+    if (group == 0) goto ISNOTFIXED;  /* Local recursion */
     for (gptr = cb->parsed_pattern; *gptr != META_END; gptr++)
       {
       if (META_CODE(*gptr) == META_BIGVALUE) gptr++;
         else if (*gptr == (META_CAPTURE | group)) break;
       }
+     
     gptrend = parsed_skip(gptr, PSKIP_KET);
     if (pptr > gptr && pptr < gptrend) goto ISNOTFIXED;  /* Local recursion */
     for (r = recurses; r != NULL; r = r->prev) if (r->groupptr == gptr) break;

testdata/testinput2

@@ -4894,4 +4894,6 @@ a)"xI
 
 /(?=.*[A-Z])/I
 
+/()(?<=(?0))/
+
 # End of testinput2 

testdata/testoutput2

@@ -15338,6 +15338,9 @@ Capturing subpattern count = 0
 May match empty string
 Subject length lower bound = 0
 
+/()(?<=(?0))/
+Failed: error 125 at offset 2: lookbehind assertion is not fixed length
+
 # End of testinput2 
 Error -63: PCRE2_ERROR_BADDATA (unknown error number)
 Error -62: bad serialized data