Reduce recursion limit for auto-possessify to reduce stack usage at compile

time. Fixes oss-fuzz issue 553.

ChangeLog

@@ -339,6 +339,10 @@ modules.
     * numbers 11, 12 & 13 should end in "th";
     * use double quotes in usage message.
     
+53. When autopossessifying, skip empty branches without recursion, to reduce 
+stack usage for the benefit of clang with -fsanitize-address, which uses huge 
+stack frames. Example pattern: /X?(R||){3335}/. Fixes oss-fuzz issue 553. 
+
 
 Version 10.22 29-July-2016
 --------------------------

src/pcre2_auto_possess.c

@@ -589,6 +589,7 @@ for(;;)
       case OP_ASSERTBACK_NOT:
       case OP_ONCE:
       case OP_ONCE_NC:
+
       /* Atomic sub-patterns and assertions can always auto-possessify their
       last iterator. However, if the group was entered as a result of checking
       a previous iterator, this is not possible. */
@@ -606,6 +607,9 @@ for(;;)
     next_code = code + GET(code, 1);
     code += PRIV(OP_lengths)[c];
 
+    /* Check each branch. We have to recurse a level for all but the last
+    branch. */
+
     while (*next_code == OP_ALT)
       {
       if (!compare_opcodes(code, utf, cb, base_list, base_end, rec_limit))
@@ -1067,7 +1071,7 @@ PCRE2_UCHAR c;
 PCRE2_SPTR end;
 PCRE2_UCHAR *repeat_opcode;
 uint32_t list[8];
-int rec_limit = 10000;
+int rec_limit = 1000;  /* Was 10,000 but clang+ASAN uses a lot of stack. */
 
 for (;;)
   {

testdata/testinput1

@@ -5826,4 +5826,9 @@ ef) x/x,mark
 /[s[:digit:]\Q\E-H]+/
     s09-H
 
+/a+(?:|b)a/
+    aaaa
+
+/X?(R||){3335}/
+
 # End of testinput1 

testdata/testoutput1

@@ -9305,4 +9305,10 @@ No match
     s09-H
  0: s09-H
 
+/a+(?:|b)a/
+    aaaa
+ 0: aaaa
+
+/X?(R||){3335}/
+
 # End of testinput1