mingw: avoid memory leak when splitting PATH

In the (admittedly, concocted) case that PATH consists only of colons, we
would leak the duplicated string.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

compat/mingw.c

@@ -1164,8 +1164,10 @@ static char **get_path_split(void)
 			++n;
 		}
 	}
-	if (!n)
+	if (!n) {
+		free(envpath);
 		return NULL;
+	}
 
 	ALLOC_ARRAY(path, n + 1);
 	p = envpath;