pcre2_match_data: allow pcre2_match_data_create_from_pattern(NULL, cx)

Prevents crashing in that case, and matches how other API calls handle
bad parameters.

It is also useful to simplify error checking in the caller side, as it
allows delaying checks for compiler failure and handling it together with
a match data creation failure.

Author: carenas

doc/pcre2_match_data_create_from_pattern.3

@@ -15,19 +15,18 @@ PCRE2 - Perl-compatible regular expressions (revised API)
 .rs
 .sp
 This function creates a new match data block for holding the result of a match.
-The first argument points to a compiled pattern. The number of capturing
-parentheses within the pattern is used to compute the number of pairs of
-offsets that are required in the match data block. These form the "output
-vector" (ovector) within the match data block, and are used to identify the
-matched string and any captured substrings when matching with
-\fBpcre2_match()\fP. If you are using \fBpcre2_dfa_match()\fP, which uses the
-output vector in a different way, you should use \fBpcre2_match_data_create()\fP
-instead of this function.
+If the first argument is NULL, this function returns NULL, otherwise The first
+argument points to a compiled pattern. The number of capturing parentheses
+within the pattern is used to compute the number of pairs of offsets that are
+required in the match data block. These form the "output vector" (ovector)
+within the match data block, and are used to identify the matched string and
+any captured substrings when matching with \fBpcre2_match()\fP. If you are
+using \fBpcre2_dfa_match()\fP, which uses the output vector in a different way, you should use \fBpcre2_match_data_create()\fP instead of this function.
 .P
 The second argument points to a general context, for custom memory management,
-or is NULL to use the same memory allocator as was used for the compiled
+or is NULL to use the same memory allocator that was used for the compiled
 pattern. The result of the function is NULL if the memory for the block could
-not be obtained.
+not be obtained or if NULL was provided as the first argument.
 .P
 There is a complete description of the PCRE2 native API in the
 .\" HREF

src/pcre2_match_data.c

@@ -81,6 +81,7 @@ PCRE2_EXP_DEFN pcre2_match_data * PCRE2_CALL_CONVENTION
 pcre2_match_data_create_from_pattern(const pcre2_code *code,
   pcre2_general_context *gcontext)
 {
+if (code == NULL) return NULL;
 if (gcontext == NULL) gcontext = (pcre2_general_context *)code;
 return pcre2_match_data_create(((const pcre2_real_code *)code)->top_bracket + 1,
   gcontext);