Fix possible negative index possibility in pcre2test.

ChangeLog

@@ -134,6 +134,8 @@ RunTest (see 4 above).
 
 34. Fix comment describing the returns from find_fixedlength().
 
+35. Fix potential negative index in pcre2test.
+
 
 Version 10.21 12-January-2016
 -----------------------------

src/pcre2test.c

@@ -3016,9 +3016,13 @@ for (;;)
       }
 
     dlen = strlen((char *)here);
-    if (here[dlen - 1] == '\n') return start;     /* End of line reached */
     here += dlen;
 
+    /* Check for end of line reached. Take care not to read data from before 
+    start (dlen will be zero for a file starting with a binary zero). */
+      
+    if (here > start && here[-1] == '\n') return start;
+
     /* If we have not read a newline when reading a file, we have either filled
     the buffer or reached the end of the file. We can detect the former by
     checking that the string fills the buffer, and the latter by feof(). If