Use assertion to ensure erroroffset return from pcre2_compile is valid

When testing the new pattern rewriting phase for regex compilation using
a fuzzer, I had a scary experience. Due to a bug in my pattern rewriting
code, pcre2_compile() could return a totally invalid erroroffset. If a
library user tried to do something with the erroroffset without checking
it for validity, in the worst case, this had the potential to lead to an
RCE vulnerability.

In case something similar ever happens again, I've added an assertion
which will make it easier to notice the problem.

src/pcre2_compile.c

@@ -11147,6 +11147,8 @@ an offset is available in the parsed pattern. */
 ptr = pattern + cb.erroroffset;
 
 HAD_EARLY_ERROR:
+PCRE2_ASSERT(ptr >= pattern); /* Ensure we don't return invalid erroroffset */
+PCRE2_ASSERT(ptr < (pattern + patlen));
 *erroroffset = ptr - pattern;
 
 HAD_ERROR: