[NES] Set shift count to zero if RAM size is zero.

Zero needs to be special cased, since the equation that computes the
shift count for a given size doesn't work with zero.

Fixes #98.

mos-platform/nes/ines-header.ld

@@ -35,17 +35,21 @@ BYTE(
 )
 
 BYTE(
-  ((__prg_ram_size_raw ? __prg_ram_size_raw :
-                         LOG2CEIL(__prg_ram_size*1024)-6) & 0xf) << 0 |
-  ((__prg_nvram_size_raw ? __prg_nvram_size_raw :
-                           LOG2CEIL(__prg_nvram_size*1024)-6) & 0xf) << 4
+  ((__prg_ram_size_raw ?
+      __prg_ram_size_raw :
+      __prg_ram_size ? LOG2CEIL(__prg_ram_size*1024)-6 : 0) & 0xf) << 0 |
+  ((__prg_nvram_size_raw ?
+      __prg_nvram_size_raw :
+      __prg_nvram_size ? LOG2CEIL(__prg_nvram_size*1024)-6 : 0) & 0xf) << 4
 )
 
 BYTE(
-  ((__chr_ram_size_raw ? __chr_ram_size_raw :
-                         LOG2CEIL(__chr_ram_size*1024)-6) & 0xf) << 0 |
-  ((__chr_nvram_size_raw ? __chr_nvram_size_raw :
-                           LOG2CEIL(__chr_nvram_size*1024)-6) & 0xf) << 4
+  ((__chr_ram_size_raw ?
+      __chr_ram_size_raw :
+      __chr_ram_size ? LOG2CEIL(__chr_ram_size*1024)-6 : 0) & 0xf) << 0 |
+  ((__chr_nvram_size_raw ?
+      __chr_nvram_size_raw :
+      __chr_nvram_size ? LOG2CEIL(__chr_nvram_size*1024)-6 : 0) & 0xf) << 4
 )
 
 BYTE(__timing & 3)