jit: fail early in ffcps_* if subject shorter than offs1 (#175)

FF_FUN would try loading a vector from an invalid address
triggering a crash.

Add the same check that is done in the x86/s390x implementations
and that was missing from the original code.

Fixes: #86

Author: carenas

src/pcre2_jit_neon_inc.h

@@ -183,6 +183,8 @@ restart:;
 #endif
 
 #if defined(FFCPS)
+if (str_ptr >= str_end)
+  return NULL;
 sljit_u8 *p1 = str_ptr - diff;
 #endif
 sljit_s32 align_offset = ((uint64_t)str_ptr & 0xf);