fix some compiler warnings

various false positives from gcc -O3 flow analysis

ChangeLog

@@ -3,11 +3,12 @@
 - limit n_thr on tiny images
 - don't exit() on memleak detected, just warn
 - add "autocrop" option to openslide load
-- argh fix affine, again
-- pngsave in interlaced mode make a copy of the whole image
-- vipsthumbnail shrinks to 1/2 window_size
+- argh fix affine, again, there were sometimes black bars with nohalo and the
+  vips8 interface
+- pngsave in interlaced mode makes a copy of the image, so it's always seq 
+- vipsthumbnail shrinks to 1/2 window_size: faster and better quality
 - vipsthumbnail defaults to bicubic + nosharpen
-- better rounding behaviour for fixed-point bicubic
+- better rounding behaviour for fixed-point bicubic reduces noise
 
 4/7/14 started 7.40.4
 - fix vips_rawsave_fd(), thanks aferrero2707 

libvips/convolution/spcor.c

@@ -262,6 +262,7 @@ vips_spcor_correlation( VipsCorrelation *correlation,
 
 				default:
 					g_assert( 0 );
+					return; 
 				}
 
 				c2 = sqrt( sum2 );

libvips/foreign/radiance.c

@@ -827,6 +827,11 @@ scanline_write( COLR *scanline, int width, FILE *fp )
 
 	for( i = 0; i < 4; i++ ) {
 		for( j = 0; j < width; ) {
+			/* Not needed, but keeps gcc used-before-set wsrning
+			 * quiet.
+			 */
+			cnt = 1;
+
 			/* Set beg / cnt to the start and length of the next 
 			 * run longer than MINRUN.
 			 */

libvips/foreign/tiff2vips.c

@@ -1053,7 +1053,8 @@ parse_header( ReadTiff *rtiff, VipsImage *out )
 	if( tfexists( rtiff->tiff, TIFFTAG_PLANARCONFIG ) ) {
 		int v; 
 
-		tfget16( rtiff->tiff, TIFFTAG_PLANARCONFIG, &v );
+		if( !tfget16( rtiff->tiff, TIFFTAG_PLANARCONFIG, &v ) )
+			return( -1 );
 		if( v == PLANARCONFIG_SEPARATE )
 			rtiff->separate = TRUE; 
 	}

libvips/iofuncs/util.c

@@ -1599,8 +1599,7 @@ vips_enum_from_nick( const char *domain, GType type, const char *nick )
 #define BIGBUF (10000)
 
 /* Scan @buf for the first "%ns" (eg. "%12s") and substitute the 
- * lowest-numbered one for @sub. @buf is @len bytes
- * in size.
+ * lowest-numbered one for @sub. @buf is @len bytes in size.
  *
  * If there are no %ns, use the first %s.
  */
@@ -1620,6 +1619,7 @@ vips__substitute( const char *domain, char *buf, size_t len, char *sub )
 
 	lowest_n = -1;
 	sub_start = NULL;
+	sub_end = NULL;
 	for( p = buf; (p = strchr( p, '%' )); p++ )  
 		if( isdigit( p[1] ) ) {
 			char *q;