Add ERR_clear_last_mark()

This allows callers to set a mark, and then clear it without removing
the errors.  Useful in case an error is encountered that should be
returned up the call stack.
Reviewed-by: NAndy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4094)

CHANGES

@@ -9,6 +9,10 @@
 
  Changes between 1.1.0f and 1.1.1 [xx XXX xxxx]
 
+  *) Add ERR_clear_last_mark(), to allow callers to clear the last mark
+     without clearing the errors.
+     [Richard Levitte]
+
   *) Add "atfork" functions.  If building on a system that without
      pthreads, see doc/man3/OPENSSL_fork_prepare.pod for application
      requirements.  The RAND facility now uses/requires this.

crypto/err/err.c

@@ -812,3 +812,26 @@ int ERR_pop_to_mark(void)
     es->err_flags[es->top] &= ~ERR_FLAG_MARK;
     return 1;
 }
+
+int ERR_clear_last_mark(void)
+{
+    ERR_STATE *es;
+    int top;
+
+    es = ERR_get_state();
+    if (es == NULL)
+        return 0;
+
+    top = es->top;
+    while (es->bottom != top
+           && (es->err_flags[top] & ERR_FLAG_MARK) == 0) {
+        top -= 1;
+        if (top == -1)
+            top = ERR_NUM_ERRORS - 1;
+    }
+
+    if (es->bottom == top)
+        return 0;
+    es->err_flags[top] &= ~ERR_FLAG_MARK;
+    return 1;
+}

include/openssl/err.h

@@ -262,6 +262,7 @@ int ERR_get_next_error_library(void);
 
 int ERR_set_mark(void);
 int ERR_pop_to_mark(void);
+int ERR_clear_last_mark(void);
 
 #ifdef  __cplusplus
 }

util/libcrypto.num

@@ -4381,3 +4381,4 @@ ASN1_TIME_cmp_time_t                    4324	1_1_1	EXIST::FUNCTION:
 ASN1_TIME_compare                       4325	1_1_1	EXIST::FUNCTION:
 EVP_PKEY_CTX_ctrl_uint64                4326	1_1_1	EXIST::FUNCTION:
 EVP_DigestFinalXOF                      4327	1_1_1	EXIST::FUNCTION:
+ERR_clear_last_mark                     4328	1_1_1	EXIST::FUNCTION: