Use assertion to ensure erroroffset return from pcre2_compile is with…

…in bounds (#460)

When testing a patch for PCRE2, I found that due to a bug in my code,
`pcre2_compile()` could return a totally invalid error offset. In case
something similar ever happens again, I've added an assertion which will
make it easier to notice the problem.

It should be noted that the pcre2api manpage states: "Some errors are
not detected until the whole pattern has been scanned; in these cases,
the offset passed back is the length of the pattern." Since patterns are
not always null-terminated, this means that `pattern + erroroffset` may
sometimes point to uninitialized (or even unmapped) memory. However,
it is still worthwhile to guard against other unexpected values being
returned in `erroroffset`.

src/pcre2_compile.c

@@ -11201,6 +11201,8 @@ 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: