Fix crash in IsSCAssem

The test for "is this an S-C Assembler source file" tried to
dereference a null pointer when asked to examine a file with a
zero-length data fork.  The test only fires for files with type=INT
and auxType=0, so this is pretty hard to hit.  The specific failing
case had a damaged file with the appropriate file type.

Issue #42

reformat/Asm.cpp

@@ -81,7 +81,11 @@ void ReformatSCAssem::Examine(ReformatHolder* pHolder)
     long srcLen = pHolder->GetSourceLen(ReformatHolder::kPartData);
     int len;
 
-    len = *ptr;
+    if (srcLen < 1) {
+        return false;
+    }
+
+    len = *ptr;     // get length byte
     if (len == 0 || len > srcLen)
         return false;       // should return an error, really
     if (ptr[len-1] == 0x00) {

reformat/Reformat.h

@@ -208,7 +208,7 @@ public:
             for (i = 0; i < kReformatMAX; i++)
                 fApplies[part][i] = kApplicUnknown;
             fSourceBuf[part] = NULL;
-            fSourceLen[part] = NULL;
+            fSourceLen[part] = 0;
             fErrorBuf[part] = NULL;
         }
         for (i = 0; i < kReformatMAX; i++) {
@@ -356,7 +356,7 @@ private:
     char*           fNameExt;       // guaranteed non-NULL
 
     /* input goes here */
-    uint8_t*         fSourceBuf[kPartMAX];
+    uint8_t*        fSourceBuf[kPartMAX];
     long            fSourceLen[kPartMAX];
 
     char*           fErrorBuf[kPartMAX];

reformat/ReformatBase.h

@@ -139,7 +139,7 @@ private:
 
 /*
  * Abstract base class for reformatting a graphics file into a
- * device-independent bitmap..
+ * device-independent bitmap.
  */
 class ReformatGraphics: public Reformat {
 public: