Fix incorrect compilation for repeated conditionals with only one branch.

ChangeLog

@@ -66,6 +66,9 @@ fuzzer.
 being diagnosed, possibly leading to a buffer overflow. This bug was found by
 the LLVM fuzzer.
 
+18. A conditional group with only one branch has an implicit empty alternative 
+branch and must therefore be treated as potentially matching an empty string.
+
 
 Version 10.20 30-June-2015
 --------------------------

src/pcre2_compile.c

@@ -5324,6 +5324,12 @@ for (;; ptr++)
               scode += GET(scode, 1);
               }
             while (*scode == OP_ALT);
+            
+            /* A conditional group with only one branch has an implicit empty 
+            alternative branch. */
+
+            if (*bracode == OP_COND && bracode[GET(bracode,1)] != OP_ALT)
+              *bracode = OP_SCOND; 
             }
 
           /* Handle possessive quantifiers. */

testdata/testinput2

@@ -4352,4 +4352,6 @@ a random value. /Ix
 
 "[[[.\xe8Nq\xffq\xff\xe0\x2|||::Nq\xffq\xff\xe0\x6\x2|||::[[[:[::::::[[[[[::::::::[:[[[:[:::[[[[[[[[[[[[:::::::::::::::::[[.\xe8Nq\xffq\xff\xe0\x2|||::Nq\xffq\xff\xe0\x6\x2|||::[[[:[::::::[[[[[::::::::[:[[[:[:::[[[[[[[[[[[[[[:::E[[[:[:[[:[:::[[:::E[[[:[:[[:'[:::::E[[[:[::::::[[[:[[[[[[[::E[[[:[::::::[[[:[[[[[[[[:[[::[::::[[:::::::[[:[[[[[[[:[[::[:[[:[~"
 
+/()(?(R)0)*+/B
+
 # End of testinput2 

testdata/testoutput2

@@ -14537,4 +14537,20 @@ Subject length lower bound = 2
 "[[[.\xe8Nq\xffq\xff\xe0\x2|||::Nq\xffq\xff\xe0\x6\x2|||::[[[:[::::::[[[[[::::::::[:[[[:[:::[[[[[[[[[[[[:::::::::::::::::[[.\xe8Nq\xffq\xff\xe0\x2|||::Nq\xffq\xff\xe0\x6\x2|||::[[[:[::::::[[[[[::::::::[:[[[:[:::[[[[[[[[[[[[[[:::E[[[:[:[[:[:::[[:::E[[[:[:[[:'[:::::E[[[:[::::::[[[:[[[[[[[::E[[[:[::::::[[[:[[[[[[[[:[[::[::::[[:::::::[[:[[[[[[[:[[::[:[[:[~"
 Failed: error 106 at offset 353: missing terminating ] for character class
 
+/()(?(R)0)*+/B
+------------------------------------------------------------------
+        Bra
+        CBra 1
+        Ket
+        Braposzero
+        SBraPos
+        SCond
+        Cond recurse any
+        0
+        Ket
+        KetRpos
+        Ket
+        End
+------------------------------------------------------------------
+
 # End of testinput2