pcre2_string_utils: avoid segfault with strlen(NULL)

As the first step towards allowing a NULL string to represent an
empty one, and as an alternative fix for the failures addressed
by PCRE2Project#53

Author: carenas

src/pcre2_string_utils.c

@@ -209,6 +209,7 @@ PCRE2_SIZE
 PRIV(strlen)(PCRE2_SPTR str)
 {
 PCRE2_SIZE c = 0;
+if (str == NULL) return 0;
 while (*str++ != 0) c++;
 return c;
 }