fix loop with malformed ppm

skip_line in ppm parse was not testing for EOF, so it could get stuck
with malformed files

thanks Kyle-Kyle

see https://github.com/libvips/libvips/issues/1377

.gitignore

@@ -1,3 +1,4 @@
+fuzz/
 .pytest_cache
 compile
 .pytest_cache

ChangeLog

@@ -2,6 +2,7 @@
 - better early shutdown in readers
 - don't attempt to save large XMP to jpeg [tnextday]
 - always fetch HEIC metadata from the main image [zhoux2016]
+- fix loop in malformed ppm [Kyle-Kyle]
 
 24/5/19 started 8.8.1
 - improve realpath() use on older libc

libvips/foreign/ppm.c

@@ -89,7 +89,10 @@
 static void 
 skip_line( FILE *fp )
 {
-        while( vips__fgetc( fp ) != '\n' )
+	int ch;
+
+        while( (ch = vips__fgetc( fp )) != '\n' && 
+		ch != EOF )
 		;
 }