Fix zero-repeated assertion-as-condition bug.

ChangeLog

@@ -53,4 +53,7 @@ matched against "abcd".
 
 8. The pcre2_substitute() function has been implemented.
 
+9. If an assertion condition was quantified with a minimum of zero (an odd
+thing to do, but it happened), SIGSEGV or other misbehaviour could occur.
+
 ****

src/pcre2_match.c

@@ -1401,8 +1401,11 @@ for (;;)
         condition = TRUE;
 
         /* Advance ecode past the assertion to the start of the first branch,
-        but adjust it so that the general choosing code below works. */
+        but adjust it so that the general choosing code below works. If the 
+        assertion has a quantifier that allows zero repeats we must skip over 
+        the BRAZERO. This is a lunatic thing to do, but somebody did! */
         
+        if (*ecode == OP_BRAZERO) ecode++; 
         ecode += GET(ecode, 1);
         while (*ecode == OP_ALT) ecode += GET(ecode, 1);
         ecode += 1 + LINK_SIZE - PRIV(OP_lengths)[condcode];

testdata/testinput2

@@ -4078,4 +4078,10 @@ a random value. /Ix
 
 # End of substitute tests 
 
+"((?=(?(?=(?(?=(?(?=())))*)))))"
+    a
+
+"(?(?=)?==)(((((((((?=)))))))))"
+    a
+
 # End of testinput2 

testdata/testoutput2

@@ -13705,4 +13705,14 @@ Failed: error -34: bad option value
 
 # End of substitute tests 
 
+"((?=(?(?=(?(?=(?(?=())))*)))))"
+    a
+ 0: 
+ 1: 
+ 2: 
+
+"(?(?=)?==)(((((((((?=)))))))))"
+    a
+No match
+
 # End of testinput2