Fix pcre2test mishandling "end before start" return with POSIX interface.

ChangeLog

@@ -317,6 +317,10 @@ just wastes time. In the UTF case it can also produce redundant entries in
 XCLASS lists caused by characters with multiple other cases and pairs of
 characters in the same "not-x" sublists.
 
+49. A pattern such as /(?=(a\K))/ can report the end of the match being before 
+its start; pcre2test was not handling this correctly when using the POSIX 
+interface (it was OK with the native interface).
+
 
 Version 10.22 29-July-2016
 --------------------------

src/pcre2test.c

@@ -6184,18 +6184,27 @@ if ((pat_patctl.control & CTL_POSIX) != 0)
       {
       if (pmatch[i].rm_so >= 0)
         {
+        PCRE2_SIZE start = pmatch[i].rm_so;
+        PCRE2_SIZE end = pmatch[i].rm_eo;
+        if (start > end)
+          {
+          start = pmatch[i].rm_eo;
+          end = pmatch[i].rm_so;
+          fprintf(outfile, "Start of matched string is beyond its end - "
+            "displaying from end to start.\n");
+          }
         fprintf(outfile, "%2d: ", (int)i);
-        PCHARSV(pp, pmatch[i].rm_so,
-          pmatch[i].rm_eo - pmatch[i].rm_so, utf, outfile);
+        PCHARSV(pp, start, end - start, utf, outfile);
         fprintf(outfile, "\n");
+
         if ((i == 0 && (dat_datctl.control & CTL_AFTERTEXT) != 0) ||
             (dat_datctl.control & CTL_ALLAFTERTEXT) != 0)
           {
           fprintf(outfile, "%2d+ ", (int)i);
-          PCHARSV(pp, pmatch[i].rm_eo, len - pmatch[i].rm_eo,
-            utf, outfile);
-          fprintf(outfile, "\n");
-          }
+          /* Note: don't use the start/end variables here because we want to
+          show the text from what is reported as the end. */
+          PCHARSV(pp, pmatch[i].rm_eo, len - pmatch[i].rm_eo, utf, outfile);
+          fprintf(outfile, "\n"); }
         }
       }
     }

testdata/testinput18

@@ -106,4 +106,7 @@
 //posix_nosub
     \=offset=70000
 
+/(?=(a\K))/
+    a
+     
 # End of testdata/testinput18

testdata/testoutput18

@@ -162,4 +162,10 @@ Failed: POSIX code 4: ? * + invalid at offset 1000001
 ** Ignored with POSIX interface: offset
 Matched with REG_NOSUB
 
+/(?=(a\K))/
+    a
+Start of matched string is beyond its end - displaying from end to start.
+ 0: a
+ 1: a
+     
 # End of testdata/testinput18