mpatch: protect against underflow in mpatch_apply (SEC)

Also caught by oss-fuzz fuzzer during development.

This defect is OVE-20180430-0002. A CVE has not been obtained as of this writing.

mercurial/mpatch.c

@@ -248,7 +248,7 b' int mpatch_apply(char *buf, const char *'
 	char *p = buf;
 
 	while (f != l->tail) {
-		if (f->start < last || f->end > len) {
+		if (f->start < last || f->end > len || last < 0) {
 			return MPATCH_ERR_INVALID_PATCH;
 		}
 		memcpy(p, orig + last, f->start - last);
@@ -258,6 +258,9 b' int mpatch_apply(char *buf, const char *'
 		p += f->len;
 		f++;
 	}
+	if (last < 0) {
+		return MPATCH_ERR_INVALID_PATCH;
+	}
 	memcpy(p, orig + last, len - last);
 	return 0;
 }