Fix signedness warnings (-Wsign-conversion, gcc)

diff --git a/lib/zip_source_filep.c b/lib/zip_source_filep.c
index c448171..4987f62 100644
--- a/lib/zip_source_filep.c
+++ b/lib/zip_source_filep.c

@@ -185,7 +185,7 @@
 	    if (S_ISREG(sb.st_mode)) {
 		ctx->supports = ZIP_SOURCE_SUPPORTS_SEEKABLE;
 
-		if (ctx->start + ctx->end > sb.st_size) {
+		if (ctx->start + ctx->end > (zip_uint64_t)sb.st_size) {
 		    zip_error_set(error, ZIP_ER_INVAL, 0);
 		    free(ctx->fname);
 		    free(ctx);
@@ -193,7 +193,7 @@
 		}
 
 		if (ctx->end == 0) {
-		    ctx->st.size = sb.st_size - ctx->start;
+		    ctx->st.size = (zip_uint64_t)sb.st_size - ctx->start;
 		    ctx->st.valid |= ZIP_STAT_SIZE;
 
 		    if (start == 0) {

diff --git a/lib/zip_source_read.c b/lib/zip_source_read.c
index 88edd65..28c0834 100644
--- a/lib/zip_source_read.c
+++ b/lib/zip_source_read.c

@@ -64,7 +64,7 @@
 		return -1;
 	    }
 	    else {
-		return bytes_read;
+		return (zip_int64_t)bytes_read;
 	    }
 	}
 

diff --git a/lib/zip_source_winzip_aes_decode.c b/lib/zip_source_winzip_aes_decode.c
index 9c8cc3f..54f695f 100644
--- a/lib/zip_source_winzip_aes_decode.c
+++ b/lib/zip_source_winzip_aes_decode.c

@@ -42,7 +42,7 @@
 #define MAX_HEADER_LENGTH (16+PWD_VER_LENGTH)
 #define HMAC_LENGTH 10
 
-static int salt_length[] = { 0, 8, 12, 16 };
+static unsigned int salt_length[] = { 0, 8, 12, 16 };
 
 struct winzip_aes {
     char *password;
@@ -139,7 +139,7 @@
 	return -1;
     }
 
-    if (_zip_fcrypt_init(ctx->mode, ctx->password, strlen(ctx->password), header, password_verification, &ctx->fcrypt_ctx) != 0) {
+    if (_zip_fcrypt_init(ctx->mode, (unsigned char *)ctx->password, strlen(ctx->password), header, password_verification, &ctx->fcrypt_ctx) != 0) {
 	zip_error_set(&ctx->error, ZIP_ER_MEMORY, 0);
 	return -1;
     }
@@ -204,7 +204,7 @@
 	    _zip_error_set_from_source(&ctx->error, src);
 	    return -1;
 	}
-	ctx->current_position += n;
+	ctx->current_position += (zip_uint64_t)n;
 
 	total = (zip_uint64_t)n;
 	for (offset = 0; offset < total; offset += ZIP_MIN(total - offset, UINT_MAX)) {

diff --git a/lib/zip_source_winzip_aes_encode.c b/lib/zip_source_winzip_aes_encode.c
index eab2964..cd30be5 100644
--- a/lib/zip_source_winzip_aes_encode.c
+++ b/lib/zip_source_winzip_aes_encode.c

@@ -42,7 +42,7 @@
 #define MAX_HEADER_LENGTH (16+PWD_VER_LENGTH)
 #define HMAC_LENGTH 10
 
-static int salt_length[] = { 0, 8, 12, 16 };
+static unsigned int salt_length[] = { 0, 8, 12, 16 };
 
 struct winzip_aes {
     char *password;
@@ -110,7 +110,7 @@
 	return -1;
     }	
     
-    if (_zip_fcrypt_init(ctx->mode, ctx->password, strlen(ctx->password), ctx->data, ctx->data+salt_length[ctx->mode], &ctx->fcrypt_ctx) != 0) {
+    if (_zip_fcrypt_init(ctx->mode, (unsigned char *)ctx->password, strlen(ctx->password), ctx->data, ctx->data+salt_length[ctx->mode], &ctx->fcrypt_ctx) != 0) {
 	zip_error_set(&ctx->error, ZIP_ER_MEMORY, 0);
 	return -1;
     }
@@ -177,7 +177,7 @@
 	}
 
 	if (ctx->eof) {
-	    return buffer_n;
+	    return (zip_int64_t)buffer_n;
 	}
 
 	if ((ret = zip_source_read(src, data, length)) < 0) {