fix false negatives with periodic needles in strstr, wcsstr, and memmem

in cases where the memorized match range from the right factor
exceeded the length of the left factor, it was wrongly treated as a
mismatch rather than a match.

issue reported by Yves Bastide.

src/string/memmem.c

@@ -120,7 +120,7 @@ static char *twoway_memmem(const unsigned char *h, const unsigned char *z, const
 		}
 		/* Compare left half */
 		for (k=ms+1; k>mem && n[k-1] == h[k-1]; k--);
-		if (k == mem) return (char *)h;
+		if (k <= mem) return (char *)h;
 		h += p;
 		mem = mem0;
 	}

src/string/strstr.c

@@ -130,7 +130,7 @@ static char *twoway_strstr(const unsigned char *h, const unsigned char *n)
 		}
 		/* Compare left half */
 		for (k=ms+1; k>mem && n[k-1] == h[k-1]; k--);
-		if (k == mem) return (char *)h;
+		if (k <= mem) return (char *)h;
 		h += p;
 		mem = mem0;
 	}

src/string/wcsstr.c

@@ -84,7 +84,7 @@ static wchar_t *twoway_wcsstr(const wchar_t *h, const wchar_t *n)
 		}
 		/* Compare left half */
 		for (k=ms+1; k>mem && n[k-1] == h[k-1]; k--);
-		if (k == mem) return (wchar_t *)h;
+		if (k <= mem) return (wchar_t *)h;
 		h += p;
 		mem = mem0;
 	}