lib: enforce consistent index() behavior with empty needle argument

On macOS, the `memmem()` function returns `NULL` instead of the expected
start of the haystack string when given a zero-length needle argument.

Add special case handling for a zero-length needle argument to ensure
that the expected offset `0` is returned on all systems.

Ref: #176
Suggested-by: Erwan MAS <[email protected]>
Signed-off-by: Jo-Philipp Wich <[email protected]>

lib.c

@@ -451,12 +451,15 @@ uc_index(uc_vm_t *vm, size_t nargs, bool right)
 					}
 					while (--p != sstr);
 				}
-				else {
+				else if (nlen > 0) {
 					p = (const char *)memmem(sstr, slen, nstr, nlen);
 
 					if (p)
 						ret = (ssize_t)(p - sstr);
 				}
+				else {
+					ret = 0;
+				}
 			}
 		}