Fix issue with \Q\E after a callout before an assertion condition.

ChangeLog

@@ -42,6 +42,9 @@ some minor bugs and Perl incompatibilities were fixed, including:
       /[[:ascii:]-z]/) now generates an error. Perl does accept this as a 
       literal, but gives a warning, so it seems best to fail it in PCRE. 
       
+  (h) An empty \Q\E sequence may appear after a callout that precedes an 
+      assertion condition (it is, of course, ignored).
+
 One effect of the refactoring is that some error numbers and messages have
 changed, and the pattern offset given for compiling errors is not always the
 right-most character that has been read. In particular, for a variable-length

src/pcre2_compile.c

@@ -2321,6 +2321,12 @@ while (ptr < ptrend)
       }
     else
       {
+      if (expect_cond_assert > 0)   /* A literal is not allowed if we are */
+        {                           /* expecting a conditional assertion, */
+        ptr--;                      /* but an empty \Q\E sequence is OK.  */
+        errorcode = ERR28;
+        goto FAILED;
+        }
       if (!inverbname && after_manual_callout-- <= 0)
         parsed_pattern = manage_callouts(thisptr, &previous_callout, options,
           parsed_pattern, cb);

testdata/testinput2

@@ -4952,4 +4952,8 @@ a)"xI
 
 /[:[:alnum:]-[[a:lnum:]+/
 
+/((?(?C'')\QX\E(?!((?(?C'')(?!X=X));=)r*X=X));=)/
+
+/((?(?C'')\Q\E(?!((?(?C'')(?!X=X));=)r*X=X));=)/
+
 # End of testinput2 

testdata/testoutput2

@@ -15434,6 +15434,11 @@ Failed: error 128 at offset 63: assertion expected after (?( or (?(?C)
 /[:[:alnum:]-[[a:lnum:]+/
 Failed: error 150 at offset 11: invalid range in character class
 
+/((?(?C'')\QX\E(?!((?(?C'')(?!X=X));=)r*X=X));=)/
+Failed: error 128 at offset 11: assertion expected after (?( or (?(?C)
+
+/((?(?C'')\Q\E(?!((?(?C'')(?!X=X));=)r*X=X));=)/
+
 # End of testinput2 
 Error -63: PCRE2_ERROR_BADDATA (unknown error number)
 Error -62: bad serialized data