Fix incorrect first matching character when a backreference with zero minimum

repeat starts a pattern (possibly after assertions).

ChangeLog

@@ -65,6 +65,11 @@ were all int variables, causing overflow when files with more than 2147483647
 lines were processed (assuming 32-bit ints). They have all been changed to 
 unsigned long ints.
 
+17. If a backreference with a minimum repeat count of zero was first in a 
+pattern, apart from assertions, an incorrect first matching character could be 
+recorded. For example, for the pattern /(?=(a))\1?b/, "b" was incorrectly set 
+as the first character of a match.
+
 
 Version 10.30 14-August-2017
 ----------------------------

src/pcre2_compile.c

@@ -7135,7 +7135,7 @@ for (;; pptr++)
     later. */
 
     HANDLE_SINGLE_REFERENCE:
-    if (firstcuflags == REQ_UNSET) firstcuflags = REQ_NONE;
+    if (firstcuflags == REQ_UNSET) zerofirstcuflags = firstcuflags = REQ_NONE;
     *code++ = ((options & PCRE2_CASELESS) != 0)? OP_REFI : OP_REF;
     PUT2INC(code, 0, meta_arg);
 

testdata/testinput2

@@ -5375,4 +5375,14 @@ a)"xI
 
 /[\d-[:print:]]/
 
+# Perl gets the second of these wrong, giving no match.
+
+"(?<=(a))\1?b"I
+    ab
+    aaab 
+
+"(?=(a))\1?b"I
+    ab
+    aaab 
+
 # End of testinput2

testdata/testoutput2

@@ -16340,6 +16340,34 @@ Failed: error 150 at offset 3: invalid range in character class
 /[\d-[:print:]]/
 Failed: error 150 at offset 3: invalid range in character class
 
+# Perl gets the second of these wrong, giving no match.
+
+"(?<=(a))\1?b"I
+Capturing subpattern count = 1
+Max back reference = 1
+Max lookbehind = 1
+Last code unit = 'b'
+Subject length lower bound = 1
+    ab
+ 0: b
+ 1: a
+    aaab 
+ 0: ab
+ 1: a
+
+"(?=(a))\1?b"I
+Capturing subpattern count = 1
+Max back reference = 1
+Starting code units: a 
+Last code unit = 'b'
+Subject length lower bound = 1
+    ab
+ 0: ab
+ 1: a
+    aaab 
+ 0: ab
+ 1: a
+
 # End of testinput2
 Error -65: PCRE2_ERROR_BADDATA (unknown error number)
 Error -62: bad serialized data