Media: Fix logic when checking IPTC alt text meta data.

Fix two conditional checks for validity of data when importing alt text from IPTC meta data. Strictly compare the results of `strpos()` as a boolean, rather than treating 0 as false; return string if `file_get_contents()` returns false.

Props suhan2411, pbiron, alexodiy, joedolson.
Fixes #64849.

git-svn-id: https://develop.svn.wordpress.org/trunk@62027 602fd350-edb4-49c9-b593-d223f7449a82

Author: joedolson

src/wp-admin/includes/image.php

@@ -1088,11 +1088,16 @@ function wp_read_image_metadata( $file ) {
 function wp_get_image_alttext( $file ) {
 	$alt_text     = '';
 	$img_contents = file_get_contents( $file );
+
+	if ( false === $img_contents ) {
+		return $alt_text;
+	}
+
 	// Find the start and end positions of the XMP metadata.
 	$xmp_start = strpos( $img_contents, '<x:xmpmeta' );
 	$xmp_end   = strpos( $img_contents, '</x:xmpmeta>' );
 
-	if ( ! $xmp_start || ! $xmp_end ) {
+	if ( false === $xmp_start || false === $xmp_end ) {
 		// No XMP metadata found.
 		return $alt_text;
 	}