gdb: fix up a few places where a char was treated as a bool

Spotted a few places where a char is being treated as a bool.  The GDB
style is to use explicit comparisons, so fix things up.

There should be no user visible changes after this commit.

gdb/coffread.c

@@ -64,7 +64,7 @@ static const registry<objfile>::key<coff_symfile_info> coff_objfile_data_key;
 
 /* Translate an external name string into a user-visible name.  */
 #define	EXTERNAL_NAME(string, abfd) \
-  (*string && *string == bfd_get_symbol_leading_char (abfd)	\
+  (*string != '\0' && *string == bfd_get_symbol_leading_char (abfd)	\
    ? string + 1 : string)
 
 /* To be an sdb debug type, type must have at least a basic or primary

gdb/dbxread.c

@@ -486,7 +486,7 @@ record_minimal_symbol (minimal_symbol_reader &reader,
       {
 	const char *tempstring = name;
 
-	if (*tempstring
+	if (*tempstring != '\0'
 	    && *tempstring == bfd_get_symbol_leading_char (objfile->obfd.get ()))
 	  ++tempstring;
 	if (is_vtable_name (tempstring))
@@ -2255,7 +2255,7 @@ read_ofile_symtab (struct objfile *objfile, legacy_psymtab *pst)
 	    processing_gcc_compilation = 1;
 	  else if (strcmp (namestring, GCC2_COMPILED_FLAG_SYMBOL) == 0)
 	    processing_gcc_compilation = 2;
-	  if (*tempstring
+	  if (*tempstring != '\0'
 	      && *tempstring == bfd_get_symbol_leading_char (symfile_bfd))
 	    ++tempstring;
 	  if (startswith (tempstring, "__gnu_compiled"))

gdb/machoread.c

@@ -394,7 +394,7 @@ macho_resolve_oso_sym_with_minsym (struct objfile *main_objfile, asymbol *sym)
   struct bound_minimal_symbol msym;
   const char *name = sym->name;
 
-  if (*name
+  if (*name != '\0'
       && *name == bfd_get_symbol_leading_char (main_objfile->obfd.get ()))
     ++name;
   msym = lookup_minimal_symbol (name, NULL, main_objfile);