Fix #78876: Long variables cause OOM and temp files are not cleaned

We use the proper type for size calculations, which is `size_t`.

(cherry picked from commit 3c8582c)

Author: cmb69

NEWS

@@ -5,6 +5,8 @@ PHP                                                                        NEWS
 - Core:
   . Fixed bug #78875 (Long filenames cause OOM and temp files are not cleaned). 
     (CVE-2019-11048) (cmb)
+  . Fixed bug #78876 (Long variables in multipart/form-data cause OOM and temp 
+    files are not cleaned). (CVE-2019-11048) (cmb)
   . Fixed bug #79434 (PHP 7.3 and PHP-7.4 crash with NULL-pointer dereference
     on !CS constant). (Nikita)
   . Fixed bug #79477 (casting object into array creates references). (Nikita)

main/rfc1867.c

@@ -614,7 +614,7 @@ static void *php_ap_memstr(char *haystack, int haystacklen, char *needle, int ne
 }
 
 /* read until a boundary condition */
-static int multipart_buffer_read(multipart_buffer *self, char *buf, size_t bytes, int *end)
+static size_t multipart_buffer_read(multipart_buffer *self, char *buf, size_t bytes, int *end)
 {
 	size_t len, max;
 	char *bound;
@@ -653,7 +653,7 @@ static int multipart_buffer_read(multipart_buffer *self, char *buf, size_t bytes
 		self->buf_begin += len;
 	}
 
-	return (int)len;
+	return len;
 }
 
 /*
@@ -663,7 +663,7 @@ static int multipart_buffer_read(multipart_buffer *self, char *buf, size_t bytes
 static char *multipart_buffer_read_body(multipart_buffer *self, size_t *len)
 {
 	char buf[FILLUNIT], *out=NULL;
-	int total_bytes=0, read_bytes=0;
+	size_t total_bytes=0, read_bytes=0;
 
 	while((read_bytes = multipart_buffer_read(self, buf, sizeof(buf), NULL))) {
 		out = erealloc(out, total_bytes + read_bytes + 1);